Add gen-sig script

[aaronmallen]

This is adds the gen-sig script to generate signatures as well as a signatures for the project.

[alassek]

Very promising! ✨

I've wanted to marry fxn's inline annotation convention with rbs sigs for a long time, but never made much progress on it. Looks like Soutaro had a similar idea.

My code review comments are mostly just nitpicks. As for how to integrate this, probably a precommit hook. We could potentially do something like repo-sync, which only makes commits if its templates generate changes, but that would take some effort.

[aaronmallen]

@alassek how do we feel about reducing the scope of this work a bit... I'm running into some walls where things aren't documented and I'm having a hard time groking their shapes. What if we reduce the scope of this PR to incorporating the script and follow up with more refined signatures in future PRs?

[aaronmallen]

closing in favor of #194

[alassek]

@aaronmallen by no means would I expect you to annotate the whole thing. Just the documented public API would be sufficient.