gvariant: substantial rework for kdbus purposes

gvariant: substantial rework for kdbus purposes

This patch is 'squashed' version of Ryan Lortie's patches from
wip/kdbus-junk branch [1].

Main changes:
- add GVariantVectors utility struct,
- support serialising/deserialising to/from GVariantVectors,
- support for single precision floats,
- GVariantVectors serialisation tests,

[1] https://git.gnome.org/browse/glib/log/?h=wip/kdbus-junk

https://bugzilla.gnome.org/show_bug.cgi?id=721861

Change-Id: If70bd22337b5a9cddac545dce45474eba542d0de