ArrayMap is great, so how about ArraySet!

Also a few little tweaks to the ArrayMap implementation.  Note that
these are fairly parallel implementations; I looked at what I could
abstract out as a base class, but there isn't really all that much without
making the resulting code more general and thus slower.  If we kept
the data structure for ArraySet the same as map, where the array has
two values per entry, then they could probably share a lot of code.
I couldn't really bring myself to do that at this point, though...

Change-Id: I9e0dd8baa8e1febcc1033ecef61623ad59ce4aae