This updates namecoin from a legacy version from about 3 years ago
(https://github.com/namecoin/namecoin-legacy) to
the new namecoin-core.
(cherry picked from commit 8bd3664f373cb78a0526dc8a86e750f55b96420a)
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.
