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/memory.rs

@@ -1,5 +1,6 @@
+use crate::host_ref::HostRef;
 use crate::{wasm_extern_t, wasm_memorytype_t, wasm_store_t, ExternHost};
-use wasmtime::{HostRef, Memory};
+use wasmtime::Memory;
 
 #[derive(Clone)]
 #[repr(transparent)]