Add a `pkgs.dhallToNix` utility

This adds a `dhallToNix` utility which compiles expression from the Dhall
configuration language to Nix using Nix's support for "import from derivation".
The main motivation of this compiler is to allow users to carve out small typed
subsets of Nix projects.  Everything in the Dhall language (except `Double`s)
can be translated to Nix in this way, including functions.