Solver interface and permutation-based depth-first search solver

[rwhender]

Tasks

• [x] Design and implement solver interface
• [x] Implement first solver, according to outline below

Solver Outline

• Find all sequence permutations for all clues (already completed in nonogram module)
  • Maybe later change this to lazy and memoized production of all permutations using a generator? That might be faster overall.
• Use depth-first search to fill in puzzle from collection of permutations.
  • Fill in a permutation for the first row
  • Verify all column clues are still satisfiable
  • If not, back track, choose another permutation, continue
  • Repeat until solved

[rwhender]

Not a great approach overall. Moving on...