fix ROOT-6066
Cosmetics
Cleanup, cosmetics, comments.
Do as wingdk does.
Pass lvalues like declRefExpr as references into the cling::Value (ROOT-6107).

This restores ReturnSynthesizer's behavior for ValueExtractionSynthesizer.
Stream refs by streaming the ref'ed object.
Test the behavior of the ValueExtractionSythesizer for objects.

Still missing: arrays incl arrays of objects.