--shuffle : We should ensure that we really are order-independent

[koo5]

Shuffle the triples in bodies and heads in some way. Ideally, we should try all permutations of the kb and query, which means all permutations of all rules, their heads, and bodies. This would take very long on more complex test cases. Might want to determine the number of possible permutations, and then complete them in random order...
Add it to tau2.py. Move the list triple-shuffling code from pyco to tau2, to happen before the shuffling, so we can get full coverage.
add a --noshuffle testcase command (like --limit, shouldbe etc) to disallow the shuffling, in tests with builtins. Alternatively, detect builtin use automatically...