Fuzzing: Low hanging fruits

[xermicus]

I started fuzzing solang. The first thing was to just download some collection of contracts and feed them into solang without any modifications (after deduplication yields ~80'000 contracts), which led to ~150 crashes. I opened an issue for each with needed details here, to not clutter our issue tracker.

There will most likely be duplicates, but different bugs can as well lead to the same failed unwrap() or unreachable!() statement in the compiler. Which means similar looking panic messages do not necessarily imply the same cause; every issue needs its own triage. Also, as soon as the root cause is clear for an issue, please update the issue title that was automatically created to reflect the real root cause.