gio: add GBytes-based input/output stream methods

Using a caller-supplied buffer for g_input_stream_read() doesn't
translate well to the semantics of many other languages, and using a
non-refcounted buffer for read_async() and write_async() makes it
impossible to manage the memory correctly currently in
garbage-collected languages.

Fix both of these issues by adding a new set of methods that work with
GBytes objects rather than plain buffers.

https://bugzilla.gnome.org/show_bug.cgi?id=671139