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/tests/import_calling_export.rs

@@ -32,7 +32,7 @@ fn test_import_calling_export() {
         }
     }
 
-    let engine = HostRef::new(Engine::default());
+    let engine = Engine::default();
     let store = HostRef::new(Store::new(&engine));
     let wasm = wat::parse_str(WAT).unwrap();
     let module = HostRef::new(Module::new(&store, &wasm).expect("failed to create module"));

crates/api/tests/traps.rs

@@ -12,7 +12,7 @@ fn test_trap_return() -> Result<(), String> {
         }
     }
 
-    let engine = HostRef::new(Engine::default());
+    let engine = Engine::default();
     let store = HostRef::new(Store::new(&engine));
     let binary = parse_str(
         r#"