Add Gio.Async functionality

Automatic generation of `async_name` wrappers for
`name_async/name_finish` pairs and utility environment for safe
running them.  Comes even with the docs.
Simplify wrappers for GLib.MarkupParser

Even if it means that start_element has a little bit different
signature than as documented in GLib, having two side-by-side lists is
just braindead, so use single table with attributes for Lua.  Also
change error handling; simply require MarkupParser handlers to throw
error when they want to report, returning does not work any more,
because original signatures return void.