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`.
This is the test-programs crate, which builds and runs whole programs
compiled to wasm32-wasi.