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Do this by adding "contains" to our codegen type system, and
implement a special contains on the unboxedtuple type.

This makes this operation quite a lot faster, but it looks like
largely because we don't implement a couple optimizations that
we should:
- we create a new tuple object every time we hit that line
- our generic contains code goes through compare(), which returns
  a box (since "<" and friends can return non-bools), but contains
  will always return a bool, so we have a bunch of extra boxing/unboxing

We probably should separate out the contains logic from the rest of the
comparisons, since it works quite differently and doesn't
gain anything by being there.