Remove the need for HostRef<Module>

This commit continues previous work and also #708 by removing the need
to use `HostRef<Module>` in the API of the `wasmtime` crate. The API
changes performed here are:

* The `Module` type is now itself internally reference counted.
* The `Module::store` function now returns the `Store` that was used to
  create a `Module`
* Documentation for `Module` and its methods have been expanded.

crates/api/src/runtime.rs

@@ -331,6 +331,7 @@ impl Engine {
 /// ocnfiguration (see [`Config`] for more information).
 #[derive(Clone)]
 pub struct Store {
+    // FIXME(#777) should be `Arc` and this type should be thread-safe
     inner: Rc<StoreInner>,
 }