feat(serde): borsh

[dzmitry-lahoda]

PRed here because serde feature is here.

[jswrenn]

This is my first time hearing of borsh, but since it has decently widespread optional support in other crates, I'm not totally opposed to including it as an optional feature.

[dzmitry-lahoda]

This is my first time hearing of borsh, but since it has decently widespread optional support in other crates, I'm not totally opposed to including it as an optional feature.

borsh does:

1. no schema in encoded data
2. deterministic encdec
3. no waste of data on tags (important if storage of very high price, like in persistent provable systems; and if CPU is high cost too)
4. performance of code generated by macro is not limited by reusing serde crate attributes (high cost of CPU)
5. small enum discriminates and sequence prefixes encoding
6. rust first with best rust support without need of external tooling, and yet with C++/TS/python impls

These requirements eliminate protobuf, cbor, bson, bincode, etc.