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/callable.rs

@@ -43,7 +43,7 @@ use wasmtime_runtime::Export;
 /// "#)?;
 ///
 /// // Initialise environment and our module.
-/// let engine = HostRef::new(wasmtime::Engine::default());
+/// let engine = wasmtime::Engine::default();
 /// let store = HostRef::new(wasmtime::Store::new(&engine));
 /// let module = HostRef::new(wasmtime::Module::new(&store, &binary)?);
 ///