The std::iterator class is being deprecated on MSVC++,
and currently if the compilation flag /std:c++latest
is used a warning is issued in this regard if any
iterators use the class as a base class.

If an external source file being compiled includes
the repeated_field.h header, the iterator clases
RepeatedPtrIterator and RepeatedPtrOverPtrsIterator
trigger the warning.

This change solves the warning and should avoid it in
the future when the default is to remove the class.

Using typedef instead of the modern "using x = y;"
to make it also work on VS2015.