Cranelift: Remove ABICallee trait (#4701)

* Cranelift: Remove `ABICallee` trait

It has only one implementation: the `ABICalleeImpl` struct. By using that
directly we can avoid unnecessary layers of generics and abstractions as well as
a couple `Box`es that were previously putting the single implementation into a
`Box<dyn>`.

* Cranelift: Rename `ABICalleeImpl` to `AbiCallee`

* Fix comments as per review

* Rename `AbiCallee` to `Callee`

cranelift/codegen/src/isa/aarch64/inst/emit.rs

@@ -630,7 +630,7 @@ pub struct EmitState {
 }
 
 impl MachInstEmitState<Inst> for EmitState {
-    fn new(abi: &dyn ABICallee<I = Inst>) -> Self {
+    fn new(abi: &Callee<AArch64MachineDeps>) -> Self {
         EmitState {
             virtual_sp_offset: 0,
             nominal_sp_to_fp: abi.frame_size() as i64,