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/wasm.rs

@@ -662,7 +662,7 @@ pub unsafe extern "C" fn wasm_instance_new(
         let import = *imports.add(i);
         externs.push((*import).ext.clone());
     }
-    let module = &(*module).module;
+    let module = &(*module).module.borrow();
     match Instance::new(store, module, &externs) {
         Ok(instance) => {
             let instance = Box::new(wasm_instance_t {