Speed up OnDigraphs
for a digraph and a permutation
#267
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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.