Can we offer ~ without GADTs or type families?

[David Feuer]

It seems to me that equality constraints could potentially be supported by
an implementation with neither GADTs nor type families. Type families don't
really seem to have much to do with it, and GADTs are strictly heavier
(GADTs ~= ExistentialQuantification + TypeEquality).

Could we get a separate LANGUAGE pragma just for equality constraints?
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