Optimize contract class hashing

[juanbono]

Consider using mini-serde.

Another option is to use a tokenizer. The complete parsing process is not necessary for reformatting, while at the same time is one of the major costs involved. If you can get as far as producing tokens such as identifier, number, colon, you can directly output the reformatted string incrementally while processing.
A small amount of parsing may and a little structuring may still be necessary if we need to sort fields, but it should go much further than identifying objects (identifier + colon + value) and saving them in a Vec.
There are several fast tokenizers available, some in pure Rust, but also simd_json by Daniel Lemire or the one made for jsonptr in WUFFS. I think there's already a wrapper for simd_json, while jsonptr would require us to make our own.

Tasks

• optimize cairo 0 contract classes.
• optimize casm contract classes.
• optimize sierra contract classes.