Altcoins - a new category of applications

Here, I present a new level of hierarchy on Nixpkgs: the Altcoins.

The idea is to put a single cryptocurrency per file, and build them
using expressions like altcoins.bitcoin. I believe this ordering is
clearer and more maintainable that the current one.
Ok, you have a point.

> Put something like "bitcoin = altcoins.bitcoin" in top-level/all-packages.nix would be good.

Yes please do, so atribute names will be the same.