Add OSS fuzz testing for the jq filters

[ncoghlan]

In #2669 (comment) it was mentioned that there would be interest in fuzztesting jqlang itself, in addition to the existing fuzz testing of the JSON parser.

I decided to see how well AFL could do using just the jq files in the test suite as the initial seed (find tests/ -name '*.jq'), and tests/torture/input0.json as the input file.

AFL complains that the seed corpus contains redundancies, but it's still sufficient to let it start generating hundredsthousands of new test cases that exercise different paths through the jq code when running AFL from the build folder:

afl-fuzz -i fuzz-corpus-jqlang -o fuzz-results-jqlang ./jq -f @@ ../tests/torture/input0.json

(where build/fuzz-corpus-jqlang contained the test suite files found above)

I haven't actually looked at any of the results yet (AFL is still running against the head of the repo), but the summary so far suggests it will be worth a maintainer's time to try a similar exploration.