treegen.go and treegen.py together provide a way

- to commit a particular BTree topology into ZODB, and
- to generate set of random tree topologies that all correspond to particular {k->v} dict.

this will be used in upcoming ΔBtail and ΔFtail tests.

See treegen.py documentation for details.

Some preliminary history:

9eca74ec    X Teach AllStructs to emit topologies with values
1b962f03    X Restructure: found bug that it was not marking objects as modified
2139af2c    X treegen: Verify that tree actually saved to storage is what was requested
b5e39d4a    X wcfs/treegen: allstructs: Do not keep all tree structures in memory
e9c4b619    X rebuild: tests: Random testing
c32055fc    X wcfs/xbtree: ΔBtail tests += ø -> Tree; Tree -> ø
4300d88a    X wcfs/xbtreetest/treegen.py: Fix it on ZODB4