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/api/src/lib.rs

@@ -1,5 +1,6 @@
 //! Wasmtime embed API. Based on wasm-c-api.
 
+#![allow(improper_ctypes)]
 #![cfg_attr(not(feature = "std"), no_std)]
 
 mod callable;