Merge PropertyReference with VarReference

Currently:

PropertyReference:
  varref = VarReference accessors *= ObjectAccessor
;

VarReference:
  !Keyword
  ref = [GeneralVariable|TrueID|^args, statements, ~statements.names, ~statements.variables]
;

VarReference can be extended in this way:

VarReference:
  !Keyword
  ref = [GeneralVariable|TrueID|^args, statements, ~statements.names, ~statements.variables]
  accessors *= ObjectAccessor
;