Switch to cargo-fuzz

[uncomputable]

Adapt the fuzzing infrastructure to use cargo-fuzz. I keep the existing shell scripts because they turned out to be useful.

Everything should work as before, except that the fuzzing flag is producing warnings when the tests are run. Let's discuss solutions here.

[apoelstra]

Can you update the lockfiles as suggested by CI?

Everything should work as before, except that the fuzzing flag is producing warnings when the tests are run. Let's discuss solutions here.

What kind of warnings?

[uncomputable]

warning: unexpected `cfg` condition name: `fuzzing`
 --> fuzz/fuzz_targets/parse_human.rs:3:13
  |
3 | #![cfg_attr(fuzzing, no_main)]
  |             ^^^^^^^
  |
  = help: expected names are: `clippy`, `debug_assertions`, `doc`, `docsrs`, `doctest`, `feature`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
  = help: consider using a Cargo feature instead
  = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
           [lints.rust]
           unexpected_cfgs = { level = "warn", check-cfg = ['cfg(fuzzing)'] }
  = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(fuzzing)");` to the top of the `build.rs`
  = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
  = note: `#[warn(unexpected_cfgs)]` on by defaul

[uncomputable]

I fixed the normal CI, but the fuzz test CI seems to be disabled. Investigating.

[uncomputable]

Fuzzing CI works now :) I had to update the upload-artifact and download-artifact actions because of deprecation. I also updated the cache action because it will be deprecated soon.