new level builder which is vastly better than the old one:
  - deterministically comples every time
  - completes quickly every time
  - never places traps in a way that prevents the level from being solvable
  - has tweakable difficulty parameters (trapFraction, extraPathFraction, keyFraction)
  - generates consistent difficulty levels at a given knob setting - extraPathFraction = 0.0f => hardest path complexity with all paths leading through the spawn region