Fuzzing improved

So far added the devel/fuzz stuff to fuzz/. 'make check' executes the fuzzers with each corpus from .in/ as a regression test. To do local fuzzing, see fuzz/README.md (not complete yet).

Single crash/leak/slowness reproducers go into .repro (if not in .in/ already).

Each fuzzer can have a dictionary for increasing code coverage, called .dict.

Next steps involve oss-fuzz/build.sh to do static building of all prerequisites so that we use 1. latest sources and 2. build prerequisites with sanitizers as well - else MSAN would produce false positives.

devel/fuzz will be removed, also as a next step.