Fixed: prefixes regression RHS. Formula::Substitute is in-place.

* The prefix of the LHS must be added as new prefix to the RHS from the
  BAT, of course. That was missing before.
* Formula::Substitute() is now Formula::SubstituteInPlace(). That
  way the Bat::RegressOneStep() method is more memory-efficient.
  Formula::Substitute() wasn't used anywhere else.
* Renamed the variables in the generated code with leading underscores
  to avoid name clashes with BAT standard names or fluent names.