Move HostRef<T> into the C API crate

It isn't used by anything except for the C API and all of our embedder-exposed
APIs are already internally `Rc`-based, so it doesn't make sense to use with
them.

crates/c-api/src/lib.rs

@@ -11,6 +11,7 @@ mod error;
 mod r#extern;
 mod func;
 mod global;
+mod host_ref;
 mod instance;
 mod linker;
 mod memory;