Provide primitives for a sub-string type and its primitive functions.

signature SUBSTRING =
sig
  eqtype char
  eqtype string
  type substring
  val app : (char -> unit) -> substring -> unit
  val base : substring -> string * int * int
  val collate : (char * char -> order) -> substring * substring -> order
  val compare : substring * substring -> order
  val concat : substring list -> string
  val concatWith : string -> substring list -> string
  val dropl : (char -> bool) -> substring -> substring
  val dropr : (char -> bool) -> substring -> substring
  val explode : substring -> char list
  val extract : string * int * int option -> substring
  val fields : (char -> bool) -> substring -> substring list
  val first : substring -> char option
  val foldl : (char * ’a -> ’a) -> ’a -> substring -> ’a
  val foldr : (char * ’a -> ’a) -> ’a -> substring -> ’a
  val full : string -> substring
  val getc : substring -> (char * substring) option
  val isEmpty : substring -> bool
  val isPrefix : string -> substring -> bool
  val isSubstring : string -> substring -> bool
  val isSuffix : string -> substring -> bool
  val position : string -> substring -> substring * substring
  val size : substring -> int
  val slice : substring * int * int option -> substring
  val span : substring * substring -> substring
  val splitAt : substring * int -> substring * substring
  val splitl : (char -> bool) -> substring -> substring * substring
  val splitr : (char -> bool) -> substring -> substring * substring
  val string : substring -> string
  val sub : substring * int -> char
  val substring : string * int * int -> substring
  val takel : (char -> bool) -> substring -> substring
  val taker : (char -> bool) -> substring -> substring
  val tokens : (char -> bool) -> substring -> substring list
  val translate : (char -> string) -> substring -> string
  val triml : int -> substring -> substring
  val trimr : int -> substring -> substring
end