feat: organize smart contracts file structures

[Josue19-08]

A structure was created for the contracts in Soroban. Each contract (semaphore_key_pts, verifier, group) has its own subdirectory under contracts/, with its configuration in Cargo.toml and its code in src/lib.rs. https://github.com/ZencypherSolutions/semaphore-stellar-contracts/issues/17

[evgongora]

Good job so far! Just missing structuring the main Semaphore contract and organizing the Semaphore group contract

[Josue19-08]

@evgongora Could review again, please.