lib: add shortcuts for fake hashes (fakeSha256, fakeSha512)

Fake hashes can be used as placeholders for all the places, where
Nix expression requires a hash, but we don't yet have one.

This should be more convenient than following:
- echo|sha256sum, copy into clipboard, go to editor, paste into previously
  edited place
- search nixpkgs for a random package, copy it's hash to cliboard, go to
  editor, paste into previously edited place

Nix can add support for these fake hashes. In that case printed error should contain
only 1 hash, so no more problem "which of two hashes from error should I use?"

Idea by irc:Synthetica