Nix flake

[VzxPLnHqr]

No description provided.

[apoelstra]

Thank you! I am not very familiar with flakes. This will be a good starting point for me to learn. I will try to study this this morning.

[apoelstra]

Tested 30f3c98 -- the README instructions do work. Very impressive. Also cool that the rust overlay is able to read our toolchain file (and I guess, this is why we need to use the overlay rather than just grabbing rustc from nixpkgs).

Disappointed that we need 3rd party dependencies to get lib.eachDefaultSystem but whatever, not gonna litigate that here..

I think if you can

• Fix the two trailing whitespace things
• Squash the "fix default package" commit into the first commit

Then I'd be happy to merge this!

[VzxPLnHqr]

My git skills are not the greatest. I did a git rebase -i main and chose "edit" on the first commit. However there was no trailing whitespace for me to remove in flake.nix so maybe that has already been fixed somehow by a prior force-push? However, I did remove the trailing whitespace in the README, and I squashed the "fix default package" commit (but possibly into the wrong parent), so there should only be two commits now: one with the code, and one for the readme.