refactor(pop-api): create explicit types for function id and version

[peterwht]

The pop-api should use explicit types for function id and version. By relying on constants, someone can accidentally switch the order of a function id and version and there will be no error.

See here: https://github.com/r0gue-io/pop-node/blob/daan/api/pop-api/src/lib.rs#L29

[evilrobot-01]

How do you envisage this is done? An enum with variants might be nice, but would still require conversion to a u8 to pass into the func_id expected by the chain extension, which could be implemented via Into<u8>. Would need to ensure no affect on contract size tho...