Sort lazily (on read).

The lazy sort was a little bit of a wash until I also added UnsafeAdd
which doesn't do a check on insert.  Consider the base difference:

benchmark                old ns/op    new ns/op    delta
BenchmarkIntersection     83161724     89620820   +7.77%
BenchmarkUnion            69752424     73173387   +4.90%
BenchmarkSet                  1126         1029   -8.61%
BenchmarkFind                  368          396   +7.61%
BenchmarkWrite            31306875     28940942   -7.56%
BenchmarkRead                 1232         1134   -7.95%

benchmark                 old MB/s     new MB/s  speedup
BenchmarkWrite              638.84       691.06    1.08x
BenchmarkRead          16221886.60  17624951.53    1.09x

vs. the new benchmark for unsafe sets:

BenchmarkSetNoCheck	10000000	       223 ns/op

This is about 5x faster than the original Add test.  In practice, it's
a *huge* performance boost on loading large amounts of things.