Cranelift: Remove the ABICaller trait (#4711)

* Cranelift: Remove the `ABICaller` trait

It has only one implementation: the `ABICallerImpl` struct. We can just use that
directly rather than having extra, unnecessary layers of generics and abstractions.

* Cranelift: Rename `ABICallerImpl` to `Caller`

cranelift/codegen/src/isa/x64/abi.rs

@@ -24,7 +24,7 @@ static STACK_ARG_RET_SIZE_LIMIT: u64 = 128 * 1024 * 1024;
 pub(crate) type X64Callee = Callee<X64ABIMachineSpec>;
 
 /// Support for the x64 ABI from the caller side (at a callsite).
-pub(crate) type X64ABICaller = ABICallerImpl<X64ABIMachineSpec>;
+pub(crate) type X64Caller = Caller<X64ABIMachineSpec>;
 
 /// Implementation of ABI primitives for x64.
 pub struct X64ABIMachineSpec;