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/abi.rs

@@ -21,7 +21,7 @@ use smallvec::{smallvec, SmallVec};
 // these ABIs are very similar.
 
 /// Support for the AArch64 ABI from the callee side (within a function body).
-pub(crate) type AArch64ABICallee = ABICalleeImpl<AArch64MachineDeps>;
+pub(crate) type AArch64Callee = Callee<AArch64MachineDeps>;
 
 /// Support for the AArch64 ABI from the caller side (at a callsite).
 pub(crate) type AArch64ABICaller = ABICallerImpl<AArch64MachineDeps>;
@@ -65,7 +65,7 @@ fn saved_reg_stack_size(
 
 /// AArch64-specific ABI behavior. This struct just serves as an implementation
 /// point for the trait; it is never actually instantiated.
-pub(crate) struct AArch64MachineDeps;
+pub struct AArch64MachineDeps;
 
 impl IsaFlags for aarch64_settings::Flags {}