[mypyc] Add write_i16_le and read_i16_le to librt.strings

[JukkaL]

These are experimental optimized primitives for reading/writing 16-bit binary integers. I'll later add support for other integer sizes and floats, and big endian formats.

The main optimization here is to inline the hot paths of these functions. Also the api is optimized for performance and not really ergonomics, though I don't think the ergonomics are particularly bad. When reading, the caller needs to keep track of the index to the bytes object, and negative indexes aren't supported, but the latter doesn't seem really useful for these kinds of primitives.

In a microbenchmark, these can be dozens of times faster than using stdlib struct (when compiled).

It may still be possible to further optimize performance, but these seem already pretty good.

I used coding agent assist but through small human reviewed increments.

[github-actions]

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