core: Add API to convert checksum -> csum in place

We already have the opposite, and this will be used in some
places to avoid a malloc.

https://bugzilla.gnome.org/show_bug.cgi?id=706031
libotutil: Make use of GBytes in ot_variant_read()

This is just cleaner; we avoid using GObject data, etc.

https://bugzilla.gnome.org/show_bug.cgi?id=706031