Add comments for each public methods and improve Gateway program organization

[lumtis]

Looking at some existing resources:

https://github.com/jup-ag/stabble-amm-sdk/tree/main/programs/stable-swap/src
https://github.com/solana-developers/program-examples/tree/main/basics/counter/native/program/src
https://github.com/solana-labs/solana-program-library/tree/master/single-pool/program/src

It seems the codebase for the gateway program can be improve and follow some pseudo standard:

state.rs for types
instruction.rs for instructions
entrypoint.rs
...