Equality of sum types is broken

[athas]

Incredibly we didn't notice until now.

-- ==
-- entry: test_sum
-- script input { mk_sum 1i32 }
-- output { false }

type sum = #none | #color i32

entry mk_sum (x: i32) : sum = #color x
entry test_sum (s: sum) = s != #none

It is internalisation that messes it up and compares even values from the unused constructors.

[athas]

Of course, a reasonable question could be if we should just ban equality on sum types in the type checker.