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.
mode argument is used to assert it works
encoding argument is used to make sure CPython uses the correct encoding
as it does not automatically use utf-8
With unicode enabled, this patch allows reading a fixed number of
characters from text-mode streams; eg file.read(5) will read 5 unicode
chars, which can made of more than 5 bytes.
For an ASCII stream (ie no chars > 127) it only needs to do 1 read. If
there are lots of non-ASCII chars in a stream, then it needs multiple
reads of the underlying object.
Adds a new test for this case. Enables unicode support by default on
unix and stmhal ports.
