Motivation for this change

afl comes bundled with this useful little malloc-wrapper which we don't currently build. Would be good to expose this to users. A few odd things about this package though where I don't know if I made the right decision:

• The source being bundled with afl, it doesn't really have a version number of its own. So I made it inherit afl's version.
• I exposed the libdislocator package in the top-level namespace because it's usable (and useful) outside the context of afl, but the nix file lives in afl's directory and directly pulls in afl.src instead of just being totally independent. Weird?
• This being a raw .so library where the intended use is through e.g. $ LD_PRELOAD=/path/to/libdislocator.so ... I didn't know how to expose this to the user's environment. The most convenient thing seemed to be to add this tiny script get-libdislocator-so to live in the user's path and allow them to do $ LD_PRELOAD=$(get-libdislocator-so) ... in the same vein as pkg-config and similar tools.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.