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This patch adds a function utf8_check() to check for a valid UTF-8 encoded
string, and calls it when constructing a str from raw bytes.  The feature
is selectable at compile time via MICROPY_PY_BUILTINS_STR_UNICODE_CHECK and
is enabled if unicode is enabled.  It costs about 110 bytes on Thumb-2, 150
bytes on Xtensa and 170 bytes on x86-64.