[ty] Simplify signature types, use them in CallableType (#18344)

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
astral-sh/ty#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.)

Author: dcreager

crates/ty_python_semantic/src/types.rs

@@ -1,11 +1,11 @@
 use super::context::InferContext;
-use super::{CallableSignature, Signature, Signatures, Type};
+use super::{Signature, Type};
 use crate::Db;
 
 mod arguments;
 mod bind;
 pub(super) use arguments::{Argument, CallArgumentTypes, CallArguments};
-pub(super) use bind::{Bindings, CallableBinding};
+pub(super) use bind::{Binding, Bindings, CallableBinding};
 
 /// Wraps a [`Bindings`] for an unsuccessful call with information about why the call was
 /// unsuccessful.