Clean up ctypes for names

[gares]

We currently use these 3 data types in the API

macro @gref :- ctype "Globnames.global_reference". % name for a global constant
macro @name :- ctype "Name.t".                     % name hint (compare as =)
string

I think we could add

macro @id :- ctype "Id.t"

and have all API adding things to the environment to require an @id, have conversion functions from the others to @id.

This new type could also be used in the module-related API, since @name is broken wrt comparison.