Optimize serialization format for 2 bytes ints

[ilevkivskyi]

This is important for serialized ASTs (think line numbers for every node). Also in this PR:

• Re-use same integer logic for str/bytes length (it is slightly less optimal, but code re-use is good).
• Remove unused field from Buffer type.
• Make format the same on 32-bit and 64-bit platforms (we still assume little-endian platform).