Return asyncio awaitable object for Gio async routines

Features/Notes:

• If no callback is given, an awaitable object is returned
• If cancellable is given, this will be used, otherwise one will be created implicitly
• user_data will be ignored if callback is not set
• If *_finish throws an exception, and the result is not fetched a message will be logged.

TODO:

• Add a asyncio mainloop implementation. Thinking of https://github.com/jhenstridge/asyncio-glib/pull/10
• Refuse Async creation for non-GLib mainloops (otherwise we leak Async objects in mass)

How it works:

• During function cache creation, async routines are detected, this means:

  • One callback of type GAsyncReadyCallback which has a closure that is GI_SCOPE_TYPE_ASYNC
  • A single cancellable parameter
  • No return/out values
  • A _finish function exists (type is not checked, but I think that is acceptable as it will raise an exception when called).
  • Save information in GIIntrospection

• If during argument dispatch, we find that the GI_SCOPE_TYPE_ASYNC callback on such a function has not been provided, we:

  • Create a new PyGIAsync instance, and keep it in the scope to return
  • Retrieve the cancellable argument
  • Pass PyGIAsync instance as user_data and set specific callback

• PyGIAsync instance implements awaitable protocol, with the following caveats/differences:

  • In python, a cancellation means the future is done immediately. This will not be the case for PyGIAsync, we will always wait for the callback method to be called.
  • Right now iteration isn't quite correct; this is only relevant when the API is used incorrectly in ways that make no sense