Comment by Bilirubino

The modules Int64 and Int32 and part of the OCaml standard library. You mentioned that it is needed dune or Janestreet in your comments to have this functionality. They are part of the standard library. Really, they are part of Ocaml core developments. Actually, for example, you even can use the library big-arrays with these types and int8, int16, signed, unsigned... even more you have platform-native signed integers (32 bits on 32-bit architectures, 64 bits on 64-bit architectures) with Bigarray.nativeint_elt as part of the standard library so all these types are there.

You also mention that Int32 and Int64 are recent, however these libraries were part of OCaml already in the 4.X versions of the compiler and standard library (now we are in the 5.3).

Note that in OCaml you can use C libraries and it is quite common to manage Int32, Int64, signed etc...