Remove the need for HostRef<Module> (#778)

* 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.

* Fix compliation of test programs harness

* Fix the python extension

* Update `CodeMemory` to be `Send + Sync`

This commit updates the `CodeMemory` type in wasmtime to be both `Send`
and `Sync` by updating the implementation of `Mmap` to not store raw
pointers. This avoids the need for an `unsafe impl` and leaves the
unsafety as it is currently.

* Fix a typo

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 {