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/tests/import_calling_export.rs

@@ -34,7 +34,7 @@ fn test_import_calling_export() {
 
     let store = Store::default();
     let wasm = wat::parse_str(WAT).unwrap();
-    let module = HostRef::new(Module::new(&store, &wasm).expect("failed to create module"));
+    let module = Module::new(&store, &wasm).expect("failed to create module");
 
     let callback = Rc::new(Callback {
         other: RefCell::new(None),