Compiler: allow to inline primitives defined in the js runtime into the generated code

[hhugo]

and move compilation of some primitives out of generate.ml into the runtime

[vouillon]

I'm wondering why this is no longer simplified.

[hhugo]

This was previously optimized by

  register_bin_prims
    [ "%int_add"; "caml_int32_add"; "caml_nativeint_add" ]
    `Pure
    (fun cx cy _ ->
      match cx, cy with
      | J.EBin (J.Minus, cz, J.ENum n), J.ENum m ->
          to_int (J.EBin (J.Plus, cz, J.ENum (J.Num.add m (J.Num.neg n))))
      | _ -> to_int (plus_int cx cy));
  register_bin_prims
    [ "%int_sub"; "caml_int32_sub"; "caml_nativeint_sub" ]
    `Pure
    (fun cx cy _ ->
      match cx, cy with
      | J.EBin (J.Minus, cz, J.ENum n), J.ENum m ->
          to_int (J.EBin (J.Minus, cz, J.ENum (J.Num.add n m)))
      | _ -> to_int (J.EBin (J.Minus, cx, cy)));