CSequence uses std::find, and CCollection uses the more efficient find method of the implementation type,
which requires that the implementation type be an associative container like QSet or std::set.
Modified CContainerBase::contains to use these new find methods.
for consistency with the container classes and to avoid any potential infinite recursion with MSVC2010.
See also commit:6a9065b
Also fixed a mistake in CSequence and CCollection, where Pimpl::erase was calling the wrong version of m_impl.erase
reported by Roland.
