Use artifacts/ rather than corpus/ to track fuzz inputs. corpus appears
to be a temporary directory that will be modified when minimizing test
cases using commands like `cargo fuzz cmin` or `cargo fuzz tmin`. For
this reason it's not suitable for long-term storage and is likely to
make the git commit-staging process very messy.
- Inform devs how to build an organized and useful corpora based on the
results of fuzz testing.
- Modify .gitignore to allow corpus/ tracking. Note: files in corpus/
should not be blindly committed but selectively added based on
usefulness
- Add emptyfile to zkas-decoder corpus because it caused a panic
