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/machinst/compile.rs

@@ -14,7 +14,7 @@ use regalloc2::{self, MachineEnv};
 pub fn compile<B: LowerBackend + TargetIsa>(
     f: &Function,
     b: &B,
-    abi: Box<dyn ABICallee<I = B::MInst>>,
+    abi: Callee<<<B as LowerBackend>::MInst as MachInst>::ABIMachineSpec>,
     machine_env: &MachineEnv,
     emit_info: <B::MInst as MachInstEmit>::Info,
 ) -> CodegenResult<(VCode<B::MInst>, regalloc2::Output)> {