New primitives:

* `sub' to subtract two numbers.
* `stringLength' to get the length of a string.
* `substring' to get a substring of a string.  These should be enough
  to allow most string operations to be expressed.

tests/lang/eval-okay-substring.nix

@@ -0,0 +1,19 @@
+with builtins;
+
+let
+
+  s = "foobar";
+
+in
+
+substring 1 2 s
++ "x"
++ substring 0 (stringLength s) s
++ "y"
++ substring 3 100 s
++ "z"
++ substring 2 (sub (stringLength s) 3) s
++ "a"
++ substring 3 0 s
++ "b"
++ substring 3 1 s