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/test-programs/tests/wasm_tests/runtime.rs

@@ -15,7 +15,7 @@ pub fn instantiate(data: &[u8], bin_name: &str, workspace: Option<&Path>) -> any
 
     let mut config = Config::new();
     config.flags(settings::Flags::new(flag_builder));
-    let engine = HostRef::new(Engine::new(&config));
+    let engine = Engine::new(&config);
     let store = HostRef::new(Store::new(&engine));
 
     let global_exports = store.borrow().global_exports().clone();