Add config subcommand

[smoelius]

This PR adds a subcommand config to manage building AFL++. The intent is to decouple the cargo-afl installation from the Rust toolchain used to install it.

No longer will it be necessary to reinstall cargo-afl following a rustup update. Instead, the following will be possible:

cargo install cargo-afl          # Install cargo-afl and build AFL++ for the default toolchain
rustup update                    # Update the default toolchain
cargo afl config --build --force # Rebuild AFL++ for the default toolchain

Another example:

cargo +nightly install cargo-afl # Install cargo-afl and build AFL++ using the nightly toolchain
cargo afl config --build         # Build AFL++ for the default (e.g., stable) toolchain

Note that the cargo-afl installation is still tied to a particular version of AFLplusplus (currently, commit AFLplusplus/AFLplusplus@7e67dc9).

Some additional details on this PR's idea are here: AFLplusplus/AFLplusplus#1907

[smoelius]

I would love to have feedback on this idea. I'm going to leave the PR open until at least December 4, 2023 (about two weeks from now),

[brunoproduit]

Tested this yesterday and had no surprises. The cmplog example still works too 🎉

[smoelius]

Thank you, @brunoproduit!