Procedural macros for starknet-core

This crate provides procedural macros for deriving the Encode and Decode traits from starknet-core. This allows defining a type like:

#[derive(Debug, PartialEq, Eq, Decode, Encode)]
struct CairoType {
    a: Felt,
    b: U256,
    c: bool,
}

and using the ::encode() and ::decode() methods, without manually implementing the corresponding traits.