Migrate hand-written signatures into inline RBS comments

[tomoasleep]

This PR migrates the hand-written RBS signatures in the sig/rbs directory to inline RBS comments.

For reviewers

In each commit, I have converted the hand-written RBS files to inline RBS comments.
I recommend reviewing each commit separately.

[ParadoxV5]

+1 to eat our own dog food eventually, but because this project is still experimental (even the semantics might still change!), it might be a bit too soon to migrate?

This also reminds me on the pros and cons of checking generated files to Git/Hub.
Uploading them eliminates their setup for development and packaging alike, while a linguist-generated entry solves their pollution in the diffs and statistics.

[soutaro]

@ParadoxV5 Good point. 👍

Dog fooding is more important I think, because the project is not very big so that it won't require huge rewrite anyway.
I'm thinking to keep the generated RBS files checked in, because having the generated code would make some sense for this experimental project.