Squash improper_ctypes warnings on nightly

Lots more warnings are showing up on nightly compilers due to a recent
change. I've opened rust-lang/rust#66373 on the compiler side for this
as well.

crates/wast/src/spectest.rs

@@ -1,3 +1,5 @@
+#![allow(improper_ctypes)]
+
 use alloc::rc::Rc;
 use core::cell::RefCell;
 use cranelift_codegen::ir::types;