Remove need for HostRef<Engine> (#762)

This commit removes the need to use `HostRef<Engine>` in the Rust API.
Usage is retained in the C API in one location, but otherwise `Engine`
can always be used directly.

This is the first step of progress on #708 for the `Engine` type.
Changes here include:

* `Engine` is now `Clone`, and is documented as being cheap. It's not
  intended that cloning an engine creates a deep copy.
* `Engine` is now both `Send` and `Sync`, and asserted to be so.
* Usage of `Engine` in APIs no longer requires or uses `HostRef`.

crates/api/src/wasm.rs

@@ -778,7 +778,7 @@ pub unsafe extern "C" fn wasm_store_delete(store: *mut wasm_store_t) {
 pub unsafe extern "C" fn wasm_store_new(engine: *mut wasm_engine_t) -> *mut wasm_store_t {
     let engine = &(*engine).engine;
     let store = Box::new(wasm_store_t {
-        store: HostRef::new(Store::new(&engine)),
+        store: HostRef::new(Store::new(&engine.borrow())),
     });
     Box::into_raw(store)
 }