Add temporary file permutations.rs.

The code is horrifying at the moment, but it does work on everything I've tried.
Therefore, it's worth saving while I keep working, in case I screw it up and want
to revert back to a working state.
Allow for <T : Ord> instead of just uint, and test using a struct and string
Clean up permutations.rs code, move some tests, move TODO
Fix #23 warning; add other #23 solution, with credit to kbwt from projecteuler.net
permutations.rs: cleanups, add benchmarks, etc. Much more work needed
Further work, including cleanup and re-thinking. Tests currently broken due to new syntax.
Modify #22 to use enumerate(). That probably made it harder to read, though...
More work on permutations.rs, even more crap in the file at the moment; tests, benchmarks and other mostly temporary stuff.