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/callable.rs

@@ -43,7 +43,7 @@ use wasmtime_runtime::Export;
 ///
 /// // Initialise environment and our module.
 /// let store = wasmtime::Store::default();
-/// let module = HostRef::new(wasmtime::Module::new(&store, &binary)?);
+/// let module = wasmtime::Module::new(&store, &binary)?;
 ///
 /// // Define the type of the function we're going to call.
 /// let times_two_type = wasmtime::FuncType::new(