Stop using ECond(x,1,0) as a way to cast to boolean

[Lupus]

generate.ml emits very weird-looking code for converting to boolean with the following helper:

let bool e = J.ECond (e, one, zero)

Examples of use:

  register_bin_prim "caml_eq_float" `Pure (fun cx cy _ ->
      bool (J.EBin (J.FloatEqEq, float_val cx, float_val cy)));
  register_bin_prim "caml_neq_float" `Pure (fun cx cy _ ->
      bool (J.EBin (J.FloatNotEq, float_val cx, float_val cy)));
  register_bin_prim "caml_ge_float" `Pure (fun cx cy _ ->
      bool (J.EBin (J.FloatLe, float_val cy, float_val cx)));
  register_bin_prim "caml_le_float" `Pure (fun cx cy _ ->
      bool (J.EBin (J.FloatLe, float_val cx, float_val cy)));
  register_bin_prim "caml_gt_float" `Pure (fun cx cy _ ->
      bool (J.EBin (J.FloatLt, float_val cy, float_val cx)));
  register_bin_prim "caml_lt_float" `Pure (fun cx cy _ ->
      bool (J.EBin (J.FloatLt, float_val cx, float_val cy)));

Probably this deserves special Rehp tree node to express "cast to bool". Dumping this idea into the issue so that it's not forgotten.