Faster all_neighbors for SimpleDiGraphs

[etiennedeg]

Credits to @abhinavmehndiratta
This was part of the 2.0.0-dev branch on LightGraphs but never made it into master.
I changed it to make it even faster, so a quick review but that should be ok.

[codecov]

Codecov Report

Merging #82 (8711137) into master (761912f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #82   +/-   ##
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  Coverage   99.46%   99.46%           
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  Files         107      107           
  Lines        5556     5622   +66     
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+ Hits         5526     5592   +66     
  Misses         30       30           

[simonschoelly]

Thanks, this one makes totally sense. There is a weak assumption that vertices are always sorted incrementally for any kind of graph type, so in the future we could also discuss extending this to other types of directed graphs.

Another idea would be to use a non-allocating iterator here, although then calculating length would be a bit more difficult - either we don't calculate it and might break some user code or we precalculate length by by doing a a pass over all vertices when constructing the iterator.

[etiennedeg]

Making it an iterator was discussed but @sbromberger thought it would be inconsistent with other neighbor functions, so I kept it like this. I added the reference to the previous commit.