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/func.rs

@@ -1,3 +1,4 @@
+use crate::host_ref::HostRef;
 use crate::{wasm_extern_t, wasm_functype_t, wasm_store_t, wasm_val_t};
 use crate::{wasm_name_t, wasm_trap_t, wasmtime_error_t, ExternHost};
 use anyhow::anyhow;
@@ -5,7 +6,7 @@ use std::ffi::c_void;
 use std::panic::{self, AssertUnwindSafe};
 use std::ptr;
 use std::str;
-use wasmtime::{Caller, Extern, Func, HostRef, Trap};
+use wasmtime::{Caller, Extern, Func, Trap};
 
 #[derive(Clone)]
 #[repr(transparent)]