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/examples/gcd.rs

@@ -38,7 +38,7 @@ fn main() -> Result<()> {
     let wasm = wat::parse_str(WAT)?;
 
     // Instantiate engine and store.
-    let engine = HostRef::new(Engine::default());
+    let engine = Engine::default();
     let store = HostRef::new(Store::new(&engine));
 
     // Load a module.