GDBusMessage: fast-path encoding of fixed arrays

Instead of handling each item separately, handle the array as a whole.

https://bugzilla.gnome.org/show_bug.cgi?id=732754
GDBusMessage: fast-path decoding of fixed arrays

Instead of creating a separate GVariant for each of the 'y's in an 'ay',
use g_variant_new_fixed_array().

https://bugzilla.gnome.org/show_bug.cgi?id=732754