Simpler BFS. The upper bound is raised by repeating the BFS with new parameters.
Flag pebbles that have been used. Delete just them.
Increase initial hash size
Merge branch 'use_pure_bfs'

Use pure BFS uses a simpler BFS implementation.

- the maximum pebble number is raised by just starting the
  search from scratch with a new upper bound (last value would dominate anyway).

  In this way it is simpler to prove that some clever tricks work.

- only pebbles that have been used at least once can be removed.