Optimize the compaction algorithm for speed

[bzaar]

This will involve two passes. In pass 1, it computes the hashes for all nodes. A hash for a node cosists of the payload has plus the hashes of all children's chars plus the hashes of all child nodes, i.e. it is recursive.
In pass two it uses the hashes to compare and merge the nodes effectively.
Both oasses are bottom up, so each node is visited only twice.

[bzaar]

This is done now in version 1.2. Building a 2.5-million word Dawg now takes 5 seconds!