Handle enum types.

** Aleksey, I have some doubts about whether this will cause problems
   on the various compilers we work on.  I also wonder whether it
   implies that our arithmetic ops all need to be changed.**


[SVN r17132]

libs/mpl

@@ -1 +1 @@
-Subproject commit f2569333cebc2087190e45653a799abe02b49cd4
+Subproject commit 82f1cf3786629995604ea6042747dfc3260fe734