The old version would export two lists to a bash builder and do pairwise
processing on the bash side. In the new version we instead generate a
logic free builder on the Nix side. This is not only conceptually
simpler but reduces the amount of code and intermediate values.
`head -cNUM ... | tr -dc SET` might generate output containing fewer
than NUM characters. Given the limited alphabet, this could result in a
fairly weak passphrase. The construction `tr </dev/urandom | head
-cNUM`, however, is sure to give us the full `NUM`.
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
