How to user-define a type equality constraint?

[Anthony Clayden]

> But why does this matter?

Because I want the semantics of that equality constraint, without switching
on any of these, which I don't otherwise use:

GADTs
TypeFamilies
TypeOperators

And if that means I can't use infix `~` in my constraints, I'll put up with
that. (I'd user-define a conventional class, say `TypeCast`. Could be that
has to be defined in terms of `~`, in which case I'd put that in a shim
module as the only place with those extensions.)
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