module Substring_intf:sig..end
create base ~pos ~len creates a a substring of the base sequence of
length len starting at position pos, i.e.
base.pos, base.pos + 1, ... base.pos + len - 1
It is required that: 0 <= pos 0 <= len pos + len <= length base
It does not copy the characters.
typebigstring =Bigstring.t
module type S =sig..end