Introduce non-comparable refs

[myreen]

In the source semantics, reference pointers are Loc values and these can be compared for equality:

cakeml/semantics/semanticPrimitivesScript.sml

Line 480 in d8b47ad

do_eq (Loc l1) (Loc l2) = Eq_val (l1 = l2) ∧

When we introduce thunks #1090, we will have Loc values that point to thunks. In order to represent thunks in the most space efficient way we don't want to support equality tests between thunk values.

This issue is about adding a boolean to the Loc constructor. The boolean will indicate whether the Loc can be compared or not. Thus all current Loc n values would be equivalent to Loc T n and all Loc F k values are pointers to heap elements that cannot be compared.

I propose that the semantics of do_eq is changed to:

do_eq (Loc b1 l1) (Loc b2 l2) = (if b1 ∧ b2 then Eq_val (l1 = l2) else Eq_type_error) ∧