Invert faster without pow()
Invert slightly faster still -- thanks, djb

This cuts out our lazy use of pow() for small powers,
and the redundancy of squaring up to 11 again at the end.
It also saves three multiplies (out of about 268) by
replacing powcyclo(t, p, 5) with a slightly better trick.
Merge pull request #5 from gnprice/fast-invert

Faster invert