Create a prelude

[dvberkel]

The README documents how to use this crate. The include section looks like this

use std::collections::HashMap;
use permutation_rs::group::{Group, GroupElement, Morphism};
use permutation_rs::group::special::SLPPermutation;
use permutation_rs::group::tree::SLP;
use permutation_rs::group::free::Word;
use permutation_rs::group::permutation::Permutation;

With a prelude this could look like

use permutation_rs::Prelude::*;