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/module.rs

@@ -204,7 +204,7 @@ impl Module {
         }
     }
     pub fn validate(store: &HostRef<Store>, binary: &[u8]) -> Result<()> {
-        let features = store.borrow().engine().borrow().config.features.clone();
+        let features = store.borrow().engine().config.features.clone();
         let config = ValidatingParserConfig {
             operator_config: OperatorValidatorConfig {
                 enable_threads: features.threads,