Implement some (not-yet functional) parts of a NAS decoder

Fix unnecessary .to_vec(), reword some comments

Create structs for NGAP/Byte responses and use return instead of mut ref

Move NGAP handler code into mod.rs in each directory

Add unit test for NGSetup, fix bugs and match to Open5GS behaviour

Add unit test for NGSetup, fix bugs and match to Open5GS behaviour

Implement (unverified) NGSetupResponse response builder

Use git branch for Hampi to include its fix of NGSetupRequest parsing

Create README.md

Support NGAP decoding using Hampi-based autogenerated crate