Refactor Inlining. NFC.

I'm working on some problems that require some slightly different
characteristics than our default inliner. While working on that,
I got frustrated by some of the accumulated cruft and structuring
of the inliner, so I decided to spend some time to clean it. This
doesn't change any behavior, but moves some of the code around,
drops now-unused arguments and properties and introduces a `Signature`
struct to encapsulate the (f, ft, atypes, atype) tuple we passed around
everywhere.

Author: Keno