Sparse set implementation based on https://youtu.be/PUJ_XdmSDZw?si=41ySCBvOdoNCV-zR

The main purpose is to support delete/undo operations on the set, so we can use it for backtracking.

NOTE: The code is intentionally kept close to the material in above video, even if it may not adhere to a conventional Elixir style.

The set is a permutation on 1..domain_size. Note: it's different from Knuth's implementation, where the set values are 0-based.

Options: :mapper - function of arity 2. Allows to associate elements of the set with values.