Infer eqtype as appropriate

[briangmilnes]

/// Do I get a general type when I use it as an eqtype in inference?
module EqTypeInf

// Works, of course
let aneq (#a:eqtype) (a1 a2: a) = a1 = a2

// Does not infer an eqtype for #a.
[@@expect_failure]
let aneq' #a (a1 a2: a) = a1 = a2
// (Error 19) - Subtyping check failed
// - Expected type eqtype got type Type0
// commit=8b6fce63ca91b16386d8f76e82ea87a3c109a208