fix: Broken protobuf varint functions, add tests

[aiven-anton]

About this change - What it does

Fixes #651. I have only looked at the functions locally. I'll leave this in draft until further investigation into why the code that relies on these broken fundamentals is not itself broken. There's a possibility these paths are never exercised, in which case a better course of action is to prune the code.

• Adds round-trip hypothesis tests for read_indexes + write_indexes.
• Adds round-trip hypothesis tests for read_varint + write_varint.

Both these tests fails without patching the functions.

• write_varint was seemingly entirely broken. Replace bytearray casting with .to_bytes() invocation.
• read_indexes expects first value to be size of array, adjust write_indexes to write length before writing values.
• read_indexes special-cased size == 0 for no apparent reason, this is removed as it looks broken and doesn't have symmetry in write_indexes.