Many small improvements and additions, with btree support in ESP8266 port

This release brings various improvements and additions to the core,
extended modules and the ESP8266 port, as well as enhancements to the docs.
There is now a "threshold" function in the gc module for the user to
configure the garbage collector to run earlier than usual, in order to help
reduce fragmentation of the heap.  The btree module is now available in the
ESP8266 port, and there is improved WebREPL file transfer: get file is now
non-blocking (this change requires an update of the client software).

py core:
- nlrx64.S: prefix mp_thread_get_state with an underscore on Mac
- stream: implement 2- and 3-arg write() method as an extension to CPython
- gc: implement GC running by allocation threshold, user configurable
- fix nlrthumb.c when DEBUG=1 is defined
- obj: issue a warning when str and bytes objects are compared
- mpconfig.h: fix description for MICROPY_PY_STR_BYTES_CMP_WARN
- objstrunicode: str_index_to_ptr: should handle bytes too
- objstrunicode: str_index_to_ptr: implement positive indexing properly
- stream: implement generic flush() method, in terms of C-level ioctl
- objstringio: add MP_STREAM_FLUSH ioctl and flush() method
- objstringio: implement MP_STREAM_SEEK ioctl and add seek() method
- stream: add adapter methods with POSIX-compatible signatures
- mpconfig.h: add MICROPY_STREAMS_POSIX_API setting
- extra switches to build "embedded" BerkeleyDB BTree lib
- allow to add more items for "clean" target using CLEAN_EXTRA
- objstr: make .partition()/.rpartition() methods configurable
- mpconfig.h: define MP_ALWAYSINLINE for reuse

- modussl: rename to modussl_axtls.c, to allow impl using other SSL libs
- modussl_axtls: further changes to allow alternative SSL modules
- modussl_axtls: add dummy setblocking() method
- vfs_fat: implement rmdir() method (aliases to unlink)
- modussl_axtls: use mp_stream_close() method
- modwebrepl: use mp_stream_close() method
- modbtree: check __bt_open() return value for error
- modbtree: switch to accepting stream object instead of filename
- modbtree: implement __contains__ operation
- modwebrepl: factor out "GET" iteration to write_file_chunk()
- modwebrepl: make GET_FILE operation non-blocking
- modbtree: open(): add option kwargs
- modwebsocket: add readline method
- modwebsocket: make compatible with non-default object models
- modwebsocket: use mp_rom_map_elem_t and friends
- modwebrepl: add GET_VER operation to query MicroPython version

- axtls: upgrade to axTLS 1.5.4 + MicroPython patchset
- axtls: update, fixes esp8266 build
- utils/stdout_helpers: move from minimal/uart_extra.c for reuse
- embed/abort_: implementation of abort_() function raising uPy exception
- berkeley-db-1.xx: switch to "embedded" branch
- berkeley-db-1.xx: update, sets default page size to 4096

- add test for extended arguments to stream .write() method
- extmod/machine_pinbase: skip if machine.PinBase is not available
- bytes_compare: rework test for bytes <-> str comparison
- extmod/btree1: close database at the end of test
- detailed test for subscripting unicode strings
- run-tests: make "regex'ed .exp" facility available to device tests
- io/bytesio_ext: test for .seek()/.flush() on BytesIO
- extmod/btree1: tests against in-memory DB (using io.BytesIO)
- too non-portable, rework as an example for unix port
- machine1: revamp to work with unix port (which has "umachine")
- basics: bytes/str.partition/rpartition are now optional

- fix Makefile to handle gc-sections linker flags on OS X
- use binary file translation mode for creating mpy files on windows
- fix mingw and msys2 compilation

minimal port:

unix port:
- mpthreadport: adjust minimum thread stack, and stack limit check
- cache libaxtls.a in local build dir
- disable MICROPY_GC_ALLOC_THRESHOLD for minimal build
- mpconfigport.h: include stdio.h by default
- file: implement MP_STREAM_FLUSH ioctl
- file: ioctl(): check that file is open before operations
- file: fdfile_ioctl(): fix argument to check_fd_is_open()
- file: use generic stream flush() method
- enable websocket module
- moduselect: allow poll.register(), etc. accept fd-like objects

stmhal port:
- fix malloc when used with external libraries
- make SPI NSS pin definition optional
- fix I2C mappings for STM32F429DISC board

esp8266 port:
- select axTLS for SSL implementation, following recent refactor
- moduos: add rmdir() function
- cache Xtensa-built libaxtls.a in local build dir
- dupterm_task_init() should be called before running, etc
- set GC alloc threshold to half of heap size
- decrease GC alloc threshold to quarter of heap size
- modpybuart: fix UART parity setting
- axtls_helpers: remove abort_(), now in lib/embed/
- mpconfigport.h: include sys/types.h for POSIX types definitions
- esp_mphal: implement libc's errno
- enable btree module
- eagle.rom.addr.v6.ld: add Enable_QMode symbol from SDK 2.0.0
- make APA102 driver inclusion configurable
- store firmware size as last 4 bytes of padding area
- append md5 hash to the generated binary
- modesp: add check_fw() function to check integrity of the firmware
- scripts/ include esp.check_fw() call
- flashbdev: reserve extra sysparam sector for SDK 2.0.0 compatibility
- scripts/inisetup: add commented-out call to esp.osdebug(None)
- modmachine: implement idle() function
- esp_mphal.h: add mp_hal_ticks_cpu() for reuse
- modutime: actually implement ticks_cpu()
- modmachine: implement dummy sleep() function
- tutorial/intro: reword para abou -fm dio switch
- modules/flashbdev: start filesystem at 0x90000
- esp8266.ld: increase firmware image size to 0x90000 (576K)

- add DHT to ESP8266 Quick Ref and Tutorial
- fix some spelling mistakes
- array: document array module
- library/index: include array module in ToC
- esp8266/intro: rename to "Getting started" from "Introduction"
- esp8266/intro: add troubleshooting section
- esp8266/quickref: link to installation instructions
- esp8266/tutorial/intro: add anchor for link from quickeref
- esp8266/intro: focus on hazards of unearthed power wrt electronics
- uio: mention seek()/flush() support for io.BytesIO

- logo/1bit-logo A black & white version of the logo

- http_server*: update for buffered-like streams (read line by line)
- embedding: example for embedding MicroPython in an app
- embedding: add README
- http_client*: be sure to close socket
- network: split recv- and read-based HTTP servers

- enable gcc LTO option for nlrthumb.c
This tag has no release notes.