Add benchmarks for sqrt.
Add benchmarks for sqrt.

    test p00024::noop ... bench:          90 ns/iter (+/- 8)
    test p00024::sqrt ... bench:       1,109 ns/iter (+/- 183)
    test p00053::noop ... bench:          88 ns/iter (+/- 7)
    test p00053::sqrt ... bench:       2,262 ns/iter (+/- 856)
    test p00100::noop ... bench:         113 ns/iter (+/- 14)
    test p00100::sqrt ... bench:       5,470 ns/iter (+/- 1,846)
    test p01000::noop ... bench:       1,003 ns/iter (+/- 1,257)
    test p01000::sqrt ... bench:      81,509 ns/iter (+/- 20,265)
    test p10000::noop ... bench:       1,279 ns/iter (+/- 94)
    test p10000::sqrt ... bench:   4,305,098 ns/iter (+/- 3,736,861)

cc #9

huonw/float