[m-dev.] user-defined equality on equivalence types

[Zoltan Somogyi]

At the moment, we allow user-defined equality only on discriminated union
types, but not on equivalence types. This is a pain in defining ADTs such
as sets as unsorted lists, since it requires you to wrap a functor around
the list, even though there is no runtime efficiency penalty for using a
notag type versus an equivalence type.

The reason for that restriction was that equivalence types were unfolded
early. After Rob's proposed change, that reason will no longer apply, and
I think we should then lift that restriction. The rtti changes required
will be fairly trivial.

Comments?

Zoltan.
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