GHash: introduce a "set" mode

Make hash tables start out in a mode in which they don't store
values at all, until the first insertion of a non-identical
key-value pair.

This reduces memory requirements by 1/3 when using hash tables
to store sets.

Based on a patch by Morten Welinder,
https://bugzilla.gnome.org/show_bug.cgi?id=644437