Fixes https://github.com/dhall-lang/dhall-haskell/issues/2267
`pkgs.dhallToNix` currently fails when a Dhall package is
interpolated into the input source code, like this:
```nix
let
pkgs = import <nixpkgs> { };
f = { buildDhallPackage }: buildDhallPackage {
name = "not";
code = "λ(x : Bool) → x == False";
source = true;
};
not = pkgs.dhallPackages.callPackage f {};
in
pkgs.dhallToNix "${not}/source.dhall True"
```
This is because `dhallToNix` was using `builtins.toFile`, which
does not permit inputs with interpolated derivations. However,
`pkgs.writeText` does not have this limitation, so we can switch
to using that instead.
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.
