(PUP-2858) Refactor EvaluatingParser::Transitional to EvaluatingParser

This merges the behavior of the Transitional subclass of the
EvaluatingParser into the main class, as the hybrid solution is no
longer a supported alternative.

As a consequence, all references to the
Pops::Parser::EvaluatingParser::Transitional has been changed to
just Pops::Parser::EvaluatingParser.

This commit alters no behavior.