Implement serialization and deserialization for usize type

[Fumuran]

This PR adds vint64 variable-length serialization and deserialization implementation for usize type.

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[irakliyk]

Looks good! Thank you! I left a few comments inline - the main one is about more error checking on decoding.

Also, is this the same format as what protobuf uses for variable integer encoding?

[irakliyk]

Looks great! Thank you! I left a couple of small comments inline. After these are addressed, let's squash the commits and merge.