Forest v2.1.4

Changes:

1. Performance:
   • Reimplement some internals of the packing algorithm to avoid
     consuming the string pool in documents with many trees. (We're about
     10% slower now, but (depending on the document) the memory usage
     can drop up to five times!)
2. Bugfixes:
   • Fix aggregate function count.
   • Provide a workaround for a PGF bug: for coordinate-shaped nodes,
     \positionnodelater provides dimensions with pt in a wrong
     catcode, so max y and friends did not work for such nodes.