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/global.rs

@@ -1,7 +1,8 @@
+use crate::host_ref::HostRef;
 use crate::{handle_result, wasmtime_error_t};
 use crate::{wasm_extern_t, wasm_globaltype_t, wasm_store_t, wasm_val_t, ExternHost};
 use std::ptr;
-use wasmtime::{Global, HostRef};
+use wasmtime::Global;
 
 #[derive(Clone)]
 #[repr(transparent)]