Astring exposes an alternative `String` module for OCaml. This module
tries to balance minimality and expressiveness for basic, index-free,
string processing and provides types and functions for substrings,
string sets and string maps.

Remaining compatible with the OCaml `String` module is a non-goal. The
`String` module exposed by Astring has exception safe functions,
removes deprecated and rarely used functions, alters some signatures
and names, adds a few missing functions and fully exploits OCaml's
newfound string immutability.