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When := is used in a comprehension the target variable is bound to the
parent scope, so it's either a global or a nonlocal.  Prior to this commit
that was handled by simply using the parent scope's id_info for the
target variable.  That's completely wrong because it uses the slot number
for the parent's Python stack to store the variable, rather than the slot
number for the comprehension.  This will in most cases lead to incorrect
behaviour or memory faults.

This commit fixes the scoping of the target variable by explicitly
declaring it a global or nonlocal, depending on whether the parent is the
global scope or not.  Then the id_info of the comprehension can be used to
access the target variable.  This fixes a lot of cases of using := in a
comprehension.

Code size change for this commit:

       bare-arm:    +0 +0.000%
    minimal x86:    +0 +0.000%
       unix x64:  +152 +0.019% standard
          stm32:   +96 +0.024% PYBV10
         cc3200:   +96 +0.052%
        esp8266:  +196 +0.028% GENERIC
          esp32:  +156 +0.010% GENERIC[incl +8(data)]
         mimxrt:   +96 +0.027% TEENSY40
     renesas-ra:   +88 +0.014% RA6M2_EK
            nrf:   +88 +0.048% pca10040
            rp2:  +104 +0.020% PICO
           samd:   +88 +0.033% ADAFRUIT_ITSYBITSY_M4_EXPRESS

Fixes issue #10895.
Signed-off-by: Damien George <damien@micropython.org>