Note that `dhall-nix` cannot be built on Windows due to its transitive
dependency on the `unix` package.
Also:
* Nix: Enable `-Werror` for `dhall-nix`
* Add support for `toMap` in `dhall-nix`:
\(x : { a : Bool }) -> toMap x
is translated as
x:
(kvs:
map (k:
{
mapKey = k;
mapValue = builtins.getAttr k kvs;
}) (builtins.attrNames kvs)) x