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/trap.rs

@@ -1,6 +1,7 @@
+use crate::host_ref::HostRef;
 use crate::{wasm_frame_vec_t, wasm_instance_t, wasm_name_t, wasm_store_t};
 use once_cell::unsync::OnceCell;
-use wasmtime::{HostRef, Store, Trap};
+use wasmtime::{Store, Trap};
 
 #[repr(C)]
 #[derive(Clone)]