Add nixpkgs submodule and one of the needed default.nix files
Ok, the basic strategy for these per-package nix-sandboxes for NON-hackage
packages is to use src=./.  Then, we should do 'git clean -f -d -x .' before
nix-build to get a clean source archive.

However, this runs into a problem with symlinks out of the current dir.
Thus this commit duplicates the LICENSE file that was previously symlinked.
Add nix expressions for everything up to lvish.  Add lattices & thread-local-storage to nixpkgs so that lvish can build via nix.  Success!
Merge branch 'master' of github.com:iu-parfunc/lvars