Set extends HashMap.
Assert on sadd.
Remove lists when empty.

Assert on scard.
Assert on srandmember.
Assert on smembers.
Assert on spop.
Assert on srem.
Assert on smove.
Assert on sunion.
Assert on sunionstore.
0.1.46
Assert on sinter.
Assert on sinterstore.
Assert on sdiff.
Assert on sdiffstore.
0.1.47
Slice sdiff list.
Slice sinter list.
Don't clamp on negative srandmember count.
0.1.48