Caml_string_extern (melange.Caml_string

TODO: check with String.of_char it's quite common that we have

Caml_string_extern.of_char x.[0]

It would be nice to generate code as below

x[0]

let of_char: char => string;

let get_string_unsafe: string => int => string;

let toUpperCase: string => string;

let of_int: int => base:int => string;

let slice: string => int => int => string;

let slice_rest: string => int => string;

let index_of: string => string => int;

let of_small_int_array: array(int) => string;

let of_small_int32_array: array(int32) => string;

let lastIndexOf: string => string => int;

let length: string => int;

let unsafe_get: string => int => char;

let unsafe_set: bytes => int => char => unit;

let repeat: string => int => string;