kernel: optimize OutNeighbours

[james-d-mitchell]

I'm writing some code that involves a very large number of calls to the homomorphism finding code for relatively small digraphs. One things that stood out when profiling that code was that DigraphNrVertices uses more time than maybe necessary. This PR optimises the C code for DigraphNrVertices.

In the stable-1.0 branch:

gap> D := CompleteDigraph(2);;
gap> for i in [1 .. 100000] do
> HomomorphismDigraphsFinder(D, D, fail, [], 1, 2, 0, [1, 2],
> [], [[1, 2]], [[1, 2]]);
> od;
gap> time;
1205

with this PR:

gap> D := CompleteDigraph(2);;
gap> for i in [1 .. 100000] do
> HomomorphismDigraphsFinder(D, D, fail, [], 1, 2, 0, [1, 2],
> [], [[1, 2]], [[1, 2]]);
> od;
gap> time;
1073

a modest saving, but a saving nonetheless.