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`.

src/bin/wasmtime.rs

@@ -266,7 +266,7 @@ fn main() -> Result<()> {
         .flags(settings::Flags::new(flag_builder))
         .debug_info(debug_info)
         .strategy(strategy);
-    let engine = HostRef::new(Engine::new(&config));
+    let engine = Engine::new(&config);
     let store = HostRef::new(Store::new(&engine));
 
     let mut module_registry = HashMap::new();

src/bin/wast.rs

@@ -154,7 +154,7 @@ fn main() {
     cfg.strategy(strategy)
         .flags(settings::Flags::new(flag_builder))
         .features(features);
-    let store = HostRef::new(Store::new(&HostRef::new(Engine::new(&cfg))));
+    let store = HostRef::new(Store::new(&Engine::new(&cfg)));
     let mut wast_context = WastContext::new(store);
 
     wast_context