Implementations - Rust + Mx

[virgil-serbanuta]

Needed for #12

This requires a bit of design work before implementing. We can do the following:

• Add structs and implementations to the syntax, write .rs files with implementations, load + run that. We may still need hacks for things like self.blockchain(). We may also need Rust operations on bytes.
• Implement these as Rust code written in K, then add some glue code so that these structs + implementations work properly without properly implementing structs + implementations. We may also need Rust operations on bytes.
• Write Rust code that translates code like self.blockchain() or BigUint::zero() or first_biguint + second_biguint directly in the required results or hooks.