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/extern.rs

@@ -1,6 +1,7 @@
+use crate::host_ref::HostRef;
 use crate::wasm_externkind_t;
 use crate::{wasm_externtype_t, wasm_func_t, wasm_global_t, wasm_memory_t, wasm_table_t};
-use wasmtime::{ExternType, Func, Global, HostRef, Memory, Table};
+use wasmtime::{ExternType, Func, Global, Memory, Table};
 
 #[derive(Clone)]
 pub struct wasm_extern_t {