Proposal to add a little bit more structure to rbi files

[dikond]

I've noticed that prior to #235 that for a given gem all signatures were written in a single file, e.g. activerecord.rbi. While it works fine for small gems it becomes much disorganised and unreadable when everything is put in one file. I'd like to open up PR that brings a little bit more structure to the activerecord type signatures, similar to the original gem structure:

/activerecord
  /all
    aggregation.rbi
    associations.rbi
    ...
    base.rbi
    callbacks.rbi
    ...

If you like the idea I'd open up a PR down the road.