Improve performance of some of the prime functions

benchmark                          old ns/op    new ns/op    delta
BenchmarkIsPrime                         234          239   +2.14%
BenchmarkNextPrime                      4455         4448   -0.16%
BenchmarkIsPrimeUint64                 22351        15498  -30.66%
BenchmarkNextPrimeUint64              729512       738179   +1.19%
BenchmarkIsPrimeUint16                    50            7  -84.58%
BenchmarkNextPrimeUint16                 104            5  -94.66%
BenchmarkProbablyPrimeUint32            1468         1380   -5.99%
BenchmarkProbablyPrimeUint64_32        67546        61367   -9.15%
BenchmarkProbablyPrimeBigInt_32        84458        73408  -13.08%
BenchmarkProbablyPrimeBigInt64         85681        76992  -10.14%
BenchmarkProbablyPrimeBigInt128       726201       681448   -6.16%