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.
Well,  think they are all "altcoins", because the "default coins" are the physical ones, like American dollar and Brazilian real. And all these coins are similar in concept, so putting them in a hierarchy like this is more interesting than to split in separate files of Nixpkgs tree.

But, what about to link both ideas? Put something like "bitcoin = altcoins.bitcoin" in top-level/all-packages.nix would be good.