Zero copy serialization for parts of smart contracts

[abizjak]

Some types and values (e.g., account addresses) are likely to benefit from a more efficient serialization scheme, both in code space and execution time.

This will also allow removing the reliance on alloc.

We should consider having a more precise interface to serialization that is harder to use, but allows for zero-copy deserialization for parts where it matters.

Improvements should be accompanied by data and benchmarks.