Cache machines in the Regexp.

Turns out this is where most of the time was going in the easy cases:
benchmark                         old ns/op     new ns/op     delta
BenchmarkRegexpMatchEasy0x32      5701          1328          -76.71%
BenchmarkRegexpMatchEasy0x1K      38737         33662         -13.10%
BenchmarkRegexpMatchEasy1x32      4365          1264          -71.04%
BenchmarkRegexpMatchEasy1x1K      37907         35337         -6.78%
BenchmarkRegexpMatchMediumx32     4732          245           -94.82%
BenchmarkRegexpMatchMediumx1K     50182         44875         -10.58%
BenchmarkRegexpMatchHardx32       127896        122713        -4.05%
BenchmarkRegexpMatchHardx1K       5861321       5887762       +0.45%

benchmark                         old MB/s     new MB/s     speedup
BenchmarkRegexpMatchEasy0x32      5.61         24.08        4.29x
BenchmarkRegexpMatchEasy0x1K      26.43        30.42        1.15x
BenchmarkRegexpMatchEasy1x32      7.33         25.31        3.45x
BenchmarkRegexpMatchEasy1x1K      27.01        28.98        1.07x
BenchmarkRegexpMatchMediumx32     0.21         4.07         19.38x
BenchmarkRegexpMatchMediumx1K     20.41        22.82        1.12x
BenchmarkRegexpMatchHardx32       0.25         0.26         1.04x
BenchmarkRegexpMatchHardx1K       0.17         0.17         1.00x