[bugfix] Fix two bugs in TransformedRangeView and TransformedRangeIterator

• Properly forward r-value references of functions to enable move-construction for function stored in TransformedRangeView where possible.
• Fix and test operator-> in TransformedRangeIterator. For transformations returning l-value references, the operator did a copy, such that it->foo() and (*it).foo() act on different objects. The fix is to correctly forward the l-value reference to the proxy-object returned by operator->.