Finalize serialization format prior to release

[brson]

Initial serialization landed in #219, but it is kinda sloppy and ad-hoc.

The serialization format is something that is hard to change once in production, so we should be intentional about exactly how things are serialized.

The current serialization is done with borsh, which is probably a good choice, though we might want to change it.

What concerns me more is that even with borsh, the serialization routines are not straightforward, with a number of hand-coded parts for things like struct fields, and elements within vectors of vectors. This complexity seems unavoidable because we don't actually have Rust structs representing our all of our data types, but are instead interpreting type descriptors of move types. A lot of the serialization decisions here are made because of the current structure of the runtime, with all the wrapper types like TypedMoveBorrowedRustVec etc, and the serialization format will need to be preserved even if the runtime structure changes.