Context::Iterator operator-> bug.

In Context::Iterator class the -> operator is currently not usable as it produces result of ‘operator->()’ yields non-pointer result error. This is caused by it's return value, which is Data_descriptor&. By itself it isn't an error, because -> operator has a special semantics, when it's overloaded, it reapplies itself to the result, but must return at the end it must return a pointer.
The solution could be simply changing return value from Data_descriptor& to Data_descriptor*.