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/lower.rs

@@ -279,7 +279,7 @@ fn emit_vm_call(
     let sig = libcall.signature(call_conv);
     let caller_conv = ctx.abi().call_conv();
 
-    let mut abi = X64ABICaller::from_func(&sig, &extname, dist, caller_conv, flags)?;
+    let mut abi = X64Caller::from_func(&sig, &extname, dist, caller_conv, flags)?;
 
     abi.emit_stack_pre_adjust(ctx);