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/misc/py/src/instance.rs

@@ -21,7 +21,7 @@ impl Instance {
         let py = gil.python();
         let exports = PyDict::new(py);
         let module = self.instance.borrow().module().clone();
-        for (i, e) in module.borrow().exports().iter().enumerate() {
+        for (i, e) in module.exports().iter().enumerate() {
             match e.ty() {
                 wasmtime::ExternType::Func(ft) => {
                     let mut args_types = Vec::new();