tsort: refactor and make allocations safe

[anastygnome]

This PR makes most allocations safe

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/pid-pipe (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/rm/isatty (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Note: The gnu test tests/unexpand/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/seq/seq-epipe is now passing!

[anastygnome]

ready to merge.

[sylvestre]

sorry but what is the benefit?
this isn't a trivial change to review

[anastygnome]

@sylvestre
Most changes are in the first commit and avoid panicking on allocation failures in the parser (and using memchr3_iter which is going to get more performance in newer releases)

the second commit is mostly doing the same graph related code and moving it to its own module for readability

[sylvestre]

avoid panicking on allocation failures in the parser

ok but why ?
like i don't think we have any other programs dealing with that properly
and i don't think it will happen with tsort in real life, no?

[anastygnome]

avoid panicking on allocation failures in the parser

ok but why ? like i don't think we have any other programs dealing with that properly and i don't think it will happen with tsort in real life, no?

I thought we were doing it elsewhere, my bad then, I guess we can keep the parser rework as it's cleaner as it is, though