[m-rev.] for review: Optimise digraph.compose.

[Julien Fischer]

On Thu, 2 Feb 2023, Peter Wang wrote:

> Benchmarking on randomly generated digraphs shows a speedup of 30-40%.
> The time to make dependencies in the compiler directory on my machine
> is reduced from 3.65 seconds to 2.40 seconds.
>
> library/digraph.m:
>    Rewrite digraph.compose more efficiently.

That's fine.

Julien.