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/misc/py/src/lib.rs

@@ -92,7 +92,7 @@ pub fn instantiate(
         ..Default::default()
     });
 
-    let engine = wasmtime::HostRef::new(wasmtime::Engine::new(&config));
+    let engine = wasmtime::Engine::new(&config);
     let store = wasmtime::HostRef::new(wasmtime::Store::new(&engine));
 
     let module = wasmtime::HostRef::new(wasmtime::Module::new(&store, wasm_data).map_err(err2py)?);