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.
I don't feel like bitcoin is an altcoin, i would leave bitcoin as it is, because it's also used by more users than other coins(others come and go).