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/s390x/mod.rs

@@ -58,7 +58,7 @@ impl S390xBackend {
         func: &Function,
     ) -> CodegenResult<(VCode<inst::Inst>, regalloc2::Output)> {
         let emit_info = EmitInfo::new(self.isa_flags.clone());
-        let abi = Box::new(abi::S390xABICallee::new(func, self, &self.isa_flags)?);
+        let abi = abi::S390xCallee::new(func, self, &self.isa_flags)?;
         compile::compile::<S390xBackend>(func, self, abi, &self.machine_env, emit_info)
     }
 }