meta-types.nix: Introduce enum type

      verify = v: any (func: func v) funcs;

    };

  enum =

    values:

    assert isList values && all isString values;

    {

      name = "enum<${concatStringsSep "," values}>";

      verify = v: isString v && elem v values;

    };

  record =

    fields:

    assert isAttrs fields && all isTypeDef (attrValues fields);