Put functional tests in tests/functional

I think it is bad for these reasons when `tests/` contains a mix of
functional and integration tests

 - Concepts is harder to understand, the documentation makes a good
   unit vs functional vs integration distinction, but when the
   integration tests are just two subdirs within `tests/` this is not
   clear.

 - Source filtering in the `flake.nix` is more complex. We need to
   filter out some of the dirs from `tests/`, rather than simply pick
   the dirs we want and take all of them. This is a good sign the
   structure of what we are trying to do is not matching the structure
   of the files.

With this change we have a clean:
```shell-session
$ git show 'HEAD:tests'
tree HEAD:tests

functional/
installer/
nixos/
```

tests/functional/lang/lib.nix

@@ -0,0 +1,61 @@
+with builtins;
+
+rec {
+
+  fold = op: nul: list:
+    if list == []
+    then nul
+    else op (head list) (fold op nul (tail list));
+
+  concat =
+    fold (x: y: x + y) "";
+
+  and = fold (x: y: x && y) true;
+
+  flatten = x:
+    if isList x
+    then fold (x: y: (flatten x) ++ y) [] x
+    else [x];
+
+  sum = foldl' (x: y: add x y) 0;
+
+  hasSuffix = ext: fileName:
+    let lenFileName = stringLength fileName;
+        lenExt = stringLength ext;
+    in !(lessThan lenFileName lenExt) &&
+       substring (sub lenFileName lenExt) lenFileName fileName == ext;
+
+  # Split a list at the given position.
+  splitAt = pos: list:
+    if pos == 0 then {first = []; second = list;} else
+    if list == [] then {first = []; second = [];} else
+    let res = splitAt (sub pos 1) (tail list);
+    in {first = [(head list)] ++ res.first; second = res.second;};
+
+  # Stable merge sort.
+  sortBy = comp: list:
+    if lessThan 1 (length list)
+    then
+      let
+        split = splitAt (div (length list) 2) list;
+        first = sortBy comp split.first;
+        second = sortBy comp split.second;
+      in mergeLists comp first second
+    else list;
+
+  mergeLists = comp: list1: list2:
+    if list1 == [] then list2 else
+    if list2 == [] then list1 else
+    if comp (head list2) (head list1) then [(head list2)] ++ mergeLists comp list1 (tail list2) else
+    [(head list1)] ++ mergeLists comp (tail list1) list2;
+
+  id = x: x;
+
+  const = x: y: x;
+
+  range = first: last:
+    if first > last
+      then []
+      else genList (n: first + n) (last - first + 1);
+
+}