Fix extraction of rule kernels

[szschaler]

Currently, this uses node names, which is wrong. Node names should only be used to resolve references from inside a .gts file. Instead, we should use Henshin's mappings.

[szschaler]

While doing this, should also add some tests with Henshin rules that don't have names for nodes (both in rules that are explicitly mapped and in such for which we're generating empty rules or identity rules). At the moment, this sort of thing leads to horrible crashes.