use btrfs as main OS parition type

add 4GB swap partition
closes #6
closes #7

disk-config.nix

@@ -17,13 +17,14 @@
               };
             };
             root = {
-              size = "100%";
+              end = "-4G";
               content = {
                 type = "filesystem";
-                format = "ext4";
+                format = "btrfs";
                 mountpoint = "/";
               };
             };
+            swap = { size = "100%"; };
           };
         };
       };