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/examples/hello.rs

@@ -35,7 +35,7 @@ fn main() -> Result<()> {
 
     // Compiler the `*.wasm` binary into an in-memory instance of a `Module`.
     println!("Compiling module...");
-    let module = HostRef::new(Module::new(&store, &binary).context("> Error compiling module!")?);
+    let module = Module::new(&store, &binary).context("> Error compiling module!")?;
 
     // Here we handle the imports of the module, which in this case is our
     // `HelloCallback` type and its associated implementation of `Callback.