Speed up `OnDigraphs` for a digraph and a permutation

[ChrisJefferson]

This came up from some code which was heavily using digraphs. this almost doubled the speed of the algorithm, as it turns out these GAP-level checks were dominating the run-time of OnDigraphs.

• Instead of checking if the permutation fixes the vertices, check if it is the identity (which is much cheaper, although catches less cases)

• Do not check explictly if the permutation maps the vertices of the graph to itself, do the mapping then check afterwards.

[wilfwilson]

Another solution is that we could create some NC versions of functions that are particularly performance-critical, including OnDigraphsNC.

[wilfwilson]

@ChrisJefferson I know this is a bit late (sorry) but I intend to come up with a resolution for this soon that keeps the performance improvement.

[github-actions[bot]]

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