Rename RandomGen to Generator and use a record instead of an ADT, simplifying the API

The goal here is to start using clearer names for parts of this API.
I'd like to do a PR in a bit to move the module to Random/Generator.elm
so that it'll live nicely with the existing Random library and have a
nice name.
Merge pull request #2 from evancz/master

Rename RandomGen to Generator and use a record instead of an ADT, simplifying the API