Optimize QUuid::toString() and relevant

Optimize QUuid::toString() and relevant

QUuid::toString() and QUuid(const QString &) are too slow now.

Task-number: QTBUG-19418
Reviewed-by: joao
Reviewed-by: Denis Dzyubenko
Reviewed-by: Ritt Konstantin
Reviewed-by: Robin Burchell
Reviewed-by: Richard J. Moore
(cherry picked from commit 7ce566ed82666ac08f137f4d8590ce589d42c82a)

Change-Id: I7e5bb4072f0941c20a7278a2d9766d4ef47be811
Reviewed-on: http://codereview.qt.nokia.com/166
Reviewed-by: Liang Qi <liang.qi@nokia.com>