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]

val of_char : char -> string

val get_string_unsafe : string -> int -> string

val toUpperCase : string -> string

val of_int : int -> base:int -> string

val slice : string -> int -> int -> string

val slice_rest : string -> int -> string

val index_of : string -> string -> int

val of_small_int_array : int array -> string

val of_small_int32_array : int32 array -> string

val lastIndexOf : string -> string -> int

val length : string -> int

val unsafe_get : string -> int -> char

val unsafe_set : bytes -> int -> char -> unit

val repeat : string -> int -> string