WIP: fuzzer

[my4ng]

This adds fuzzing using cargo-fuzz with the libfuzzer backend (setup).

// run fuzzer
cargo +nightly fuzz run fuzz_bump

// generate coverage.profdata (after run)
cargo +nightly fuzz cov fuzz_bump

To generate the coverage HTML report from the coverage.profdata is a bit more complicated (see).

There are still quite a few things to iron out:

• increase coverage (currently ~45% of function coverage)
• alloc_ methods are parameterised by a generic type T, which must be known at compile time. Current solution is generate a set of size/alignment pairs with macros, but this is of course hard-coded and potentially limiting. It also increases the compile time of the fuzzer significantly even with a small set.
• GitHub workflow CI