win32mbcs: add reversing wrapper for some unicode-incompatible functions. This changeset fix the problem to use win32mbcs with mercurial 2.3 or later. The problem is brought by side effect of modification of encoding.upper() (changeset 17236:9fb8312dbdbd) because upper() does not accept unicode string argument. So wrapped util.normcase() which uses upper() will fail. In other words, upper() and lower() are unicode incompatible. To fix this issue, this changeset adds new wrapper for reversed conversion (unicode to str) for lower() and upper() to use them safely.