Clean-up after reference refactoring

Submitted by Botond Baranyi

A few minor tasks were left out of the reference refactoring:

merging the Ref_base class with the Ttcn::Reference class, as it no longer serves any purpose (it was the abstract base class for the Ttcn::Reference and Ref_pard classes),
renaming the FieldAndArrayRef class, since it can now also contain function references, not just field names and array indexes,
some of the functions in Ttcn::Reference could probably be optimized (in most cases only an 'if' was added to separate the 'Ref_pard' code from the original 'Ttcn::Reference' code).

Version: 6.6.1