Make generate-id() deterministic

Rework the generate-id() function to return deterministic values. We use
a simple incrementing counter and store ids in the 'psvi' member of
nodes which was freed up by previous commits. The presence of an id is
indicated by a new "source node" flag.

This fixes long-standing problems with reproducible builds, see
https://bugzilla.gnome.org/show_bug.cgi?id=751621

This also hardens security, as the old implementation leaked the
difference between a heap and a global pointer, see
https://bugs.chromium.org/p/chromium/issues/detail?id=1356211

The old implementation could also generate the same id for dynamically
created nodes which happened to reuse the same memory. Ids for namespace
nodes were completely broken. They now use the id of the parent element
together with the hex-encoded namespace prefix.