[feature request] new roles submodule with role_id and role_names functions. #35

I suggest adding a new roles module with roles info and add at least two functions:

def role_id(role_name):
    return DESCRIPTOR.enum_types_by_name["Role"].values_by_name[role_name].number

def role_name(role_id):
    return DESCRIPTOR.enum_types_by_name["Role"].values_by_number[role_id].name

Also, It is good to have something like constants with Roles id.
No need to go to documentation to check which roles we have or if you try to remember name (and you have tips from IDE :) ).

We have such constants in ast2vec:
https://github.com/src-d/ast2vec/blob/master/ast2vec/bblfsh_roles.py#L18

Can make a PR if you approve.