Usability improvements

[conradoplg]

• Import phf and re-export it, then change the generated code to use that (e.g. openrpsee::phf::*) instead of phf directly. This allows users to only import openrpsee without having to manually import phf too
• Make the METHODS array pub(crate) instead of pub, to avoid exposing it to crate users (or make it configurable?)