Improve `CayleyDigraph` performance

[reiniscirpons]

The current CayleyDigraph method is rather inefficient, see excerpt below:

set := AsSet(G);
D := Digraph(IsImmutableDigraph,
             G,
             set,
             OnLeftInverse,
             {x, y} -> LeftQuotient(x, y) in gens);

The issue comes from using the Digraph method for this. In the current form, the check LeftQuotient(x, y) in gens is run for each element x, y in the group G. This means the final algorithm has complexity O(|G|^2). On the other hand, using something like the Todd-Coxeter or maybe even Froidure-Pin would have complexity O(|G|*|gens|) where gens is the set of generators of G that we are constructing the group with. So it would be quite useful to improve the CayleyDigraph method to use one of these algorithms instead.