add script for coverage of fuzzing

fix #137 (closed)

To use, run ./maint/with_coverage ./maint/coverage_fuzz_corpora and wait for a very long time.
coverage_fuzz_corpora is tailored for use with with_coverage.
This takes literally hours to compile without parallelism on a recent and decently powerful laptop, so I'd recommend to use it with parallelism if you have enough ram (not sure how much, but enough to run 7 compilations in parallel), and don't fear a 80+ load average. For some reason curve25519-dalek seems to not like fuzzing+coverage 🤷‍♀️

You can see result here (temporary, will disappear at some point). (Ran with arti-corpora plus a few hours worth of fuzzing)