Allow linear searches of small buckets

This isn't perfect, but it saves a lot of sorts on small additions.

100 seems a bit large for a sorting threshold, but it seemed to work out
pretty well.  It may vary by platform or hash length, but we're starting
with something here.

Benchmarks were from a 64-bit atom machine running debian

benchmark                  old ns/op    new ns/op    delta
BenchmarkContainsSmall         57782        54765   -5.22%
BenchmarkContainsMedium         8126         6962  -14.32%
BenchmarkContainsLarge        195146       187281   -4.03%
BenchmarkIntersection      734192106    798060388   +8.70%
BenchmarkUnion             426337162    526353701  +23.46%
BenchmarkSet                    6998         2879  -58.86%
BenchmarkSetNoCheck             1315         1385   +5.32%
BenchmarkFind                   1845         2226  +20.65%
BenchmarkWrite             221512177    213603748   -3.57%
BenchmarkRead                   2209         2223   +0.63%

benchmark                   old MB/s     new MB/s  speedup
BenchmarkWrite                 90.29        93.63    1.04x
BenchmarkRead             9051262.20   8996356.73    0.99x