Add fuzz harnesses for parseDerivaton and parseDump

[xokdvium]

Motivation

My very WIP tree with fuzzers for the derivation and NAR parsers. Put up for visibility motivated by #15864.

Context

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[xokdvium]

For this to work I build with -b_sanitize=undefined,address,fuzzer-no-link (as described in debugging.md).

[asymmetric]

Just for reference, these are the steps I had to follow:

nix develop .#native-clangStdenv
appendToVar mesonFlags "-Db_sanitize=address,undefined,fuzzer-no-link"
appendToVar mesonFlags "-Dlibexpr:gc=disabled"
configurePhase
buildPhase

Then the two fuzz targets can be run with eg

src/libutil-tests/fuzz/nix-meson-build-support/fuzz/fuzz-parse-dump ../src/libutil-tests/fuzz/data/parse-dump/

Maybe we can start gathering this in a fuzzing-specific document. The information is all there, but spread across a couple of documents currently.