Const for runtime constraint

It allows to have non-standard limits for non-standard runtime.
ie: js_of_ocaml use 32bits integer, and can handle
string much bigger than 16Mo.
True, the point is to make this 32 when compiling to JavaScript.
This comment is true for (apply to) the runtime distributed with the OCaml compiler.
Would you prefer the following comment ?
```
(** Size of one int, usually 31 or 63 bits. *)
```