[mypyc] Support the u8 native integer type (#15564)
This is mostly similar to `i16` that I added recently in #15464, but
there are some
differences:
* Some adjustments were needed to support unsigned integers
* Add overflow checking of literals, since it's easy to over/underflow
when using `u8` due to limited range
* Rename primitive integer types from `int16` to `i16` (etc.) to match
the user-visible types (needed to get some error messages consistent,
and it's generally nicer)
* Overall make things a bit more consistent
* Actually update `mypy_extensions` stubs
This is an unsigned type to make it easier to work with binary/bytes
data. The item values for `bytes` are unsigned 8-bit values, in
particular. This type will become much more useful once we support
packed arrays.
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Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>
