libexpr: use std::function rather than C function pointers for primops

In preparation for the C bindings, which use this.

Change-Id: Ic1c58998951e69e66a794976585a381754dfbbeb

src/libexpr/eval.hh

@@ -36,7 +36,7 @@ enum RepairFlag : bool;
 /**
  * Function that implements a primop.
  */
-typedef void (* PrimOpFun) (EvalState & state, const PosIdx pos, Value * * args, Value & v);
+typedef std::function<void (EvalState &, const PosIdx, Value **, Value &)> PrimOpFun;
 
 /**
  * Info about a primitive operation, and its implementation

src/libexpr/primops.cc

@@ -3279,8 +3279,11 @@ static void prim_sort(EvalState & state, const PosIdx pos, Value * * args, Value
            callFunction. */
         /* TODO: (layus) this is absurd. An optimisation like this
            should be outside the lambda creation */
-        if (args[0]->isPrimOp() && args[0]->primOp->fun == prim_lessThan)
-            return CompareValues(state, noPos, "while evaluating the ordering function passed to builtins.sort")(a, b);
+        if (args[0]->isPrimOp()) {
+            auto target = args[0]->primOp->fun.target<decltype(&prim_lessThan)>();
+            if (target && *target == prim_lessThan)
+                return CompareValues(state, noPos, "while evaluating the ordering function passed to builtins.sort")(a, b);
+        }
 
         Value * vs[] = {a, b};
         Value vBool;