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`.
