There were many fields in `Signature` and friends that really had more
to do with how a signature was being _used_ — how it was looked up,
details about an individual call site, etc. Those fields more properly
belong in `Bindings` and friends.
This is a pure refactoring, and should not affect any tests or ecosystem
projects.
I started on this journey in support of
https://github.com/astral-sh/ty/issues/462. It seemed worth pulling out
as a separate PR.
One major concrete benefit of this refactoring is that we can now use
`CallableSignature` directly in `CallableType`. (We can't use
`CallableSignature` directly in that `Type` variant because signatures
are not currently interned.)
452f992fbc