meson: Generate fuzzer inputs with directives

meson: Generate fuzzer inputs with directives

The lists of directives for fuzzer tests are maintained manually in the
repo. There is a tools/check-directives.sh script that runs during test
phase and reports stale directive lists.
Let's rework the script into a generator so that these directive files
are created on-the-flight and needn't be updated whenever a unit file
directives change. The scripts is rewritten in Python to get rid of gawk
dependency and each generated file is a separate meson target so that
incremental builds refresh what is just necessary (and parallelize
(negligible)).

Note: test/fuzz/fuzz-unit-file/directives-all.slice is kept since there
is not automated way to generate it (it is not covered by the check
script neither).