Test.

Added a test for three-of-a-kind. Interesting that so much
of the pair test matches what we want for three-of-a-kind.
Implementation.

More or less a copy of the pair-p cond expression. I'll
refactor later on, once I've done all of the scores.
Test.

Turns out I skipped the two-pair test. I've added that now
and have updated the score for three-of-a-kind to 4. Again,
seems like most of this implementation will be about copying
pair-p cond expression.
Implementation.

This was slightly trickier than I thought it would be. Turns
out I had to refactor two-pair-p somewhat. It now returns a
list of cards in the two pairs. Not a list of lists of cards.
Also, composing the scores is a bit different from the other
ones. Still hopeful it will be similar enough for some common
code though.