[mypyc] Add minimal BytesWriter class librt

[JukkaL]

The librt.strings.BytesWriter class allow building a bytes object from bytes or u8 components.

Simple example use case:

from librt.strings import BytesWriter

w = BytesWriter()
w.write(b'foo')
w.append(ord('!'))
print(w.getvalue())  # b'foo!'

It can be used at an alternative to bytearray or BytesIO in various use cases. The main benefit over stdlib classes is that we can provide (more) efficient mypyc primitives for various operations. The goal is to make it more efficient than either bytearray or BytesIO in many use cases, eventually.

The implementation is based on the implementation of librt.internal.WriteBuffer, but it is somewhat different, since it will support also direct indexed item access (a bit like bytearray), so just appending to the end is not the only important use case. Direct item access is not implemented yet, but I'll add more features in follow-up PRs.

This minimal implementation hasn't been optimized yet, and the API may not be final. It's experimental and only compiled if experimental features are explicitly enabled, so we can still make arbitrary API and ABI changes.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅