generalize fold_to_list, arg, right and left transforms, identity transforms for the expression generators, expr<> and ref<>

[SVN r36472]

libs/xpressive

@@ -1 +1 @@
-Subproject commit 2d137ec918cc6463d38e21a2236898d007766ca0
+Subproject commit 2219f3e5d023e70cb1a12922b92793178b34d8d0