Specs: Use "i", "j" and "k" variables in assignments

This makes the variables different from "a", "b" and "c" which are often
used as arguments of functions or blocks. This ensures that assignemnts
used as function or block bodies will translate the same independent on
the context and passed/non-passed argument.

This is not immediately needed, but it will be in the near future.
Kill Y2R::AST::Ruby::Arg class

The Y2R::AST::Ruby::Arg class only existed because I assumed that we
will need defaults for arguments. It turned out we don't, which means
that the role of this class can be easily fulfilled by
Y2R::AST::Ruby::Variable.