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/runtime/src/lib.rs

@@ -1,5 +1,6 @@
 //! Runtime library support for Wasmtime.
 
+#![allow(improper_ctypes)]
 #![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
 #![warn(unused_import_braces)]
 #![cfg_attr(feature = "std", deny(unstable_features))]