ci: add ClusterFuzzLite

[alaviss]

Add a new fuzz target to the repository. This target can be built using clang + libFuzzer by running:

make -C src/fuzz

# Alternatively, to use AFL++
CC=afl-clang-fast make -C src/fuzz

The target can be run locally with

./src/fuzz/zforth_fuzzer # -help=1 for possible flags

# To use with AFL++
afl-fuzz -i forth -o <output directory> -- ./src/fuzz/zforth_fuzzer

There is a dictionary used to improve fuzz coverage in src/fuzz/zforth_fuzzer.dict that I made with words from the primitive set, please take a look to make sure that it's correct. You can find some samples dictionaries here.

I also found some weird infinite loops that might have to do with how I'm using zForth (you can look at the failing CI to see a test case), so some review for that would be appreciated.

Added with this is a continuous fuzzing job using ClusterFuzzLite. This is the smaller version of Google's OSS-Fuzz project.

Right now there are three jobs being run:

• Daily fuzzing: A daily 1hr fuzzing job
• PR fuzzing: Fuzz all PRs for 5 mins
• Weekly coverage and corpus cleaning job