Revert "Extract `Ast::StepResult`"

This reverts commit db896618181cde9c4114c176c91685209891c3ed, keeping
the formatter API consistent between 1.x and 2.0

Conflicts:
	lib/cucumber/ast/step.rb
	lib/cucumber/ast/step_invocation.rb
	lib/cucumber/ast/step_result.rb
	lib/cucumber/ast/tree_walker.rb
	spec/cucumber/formatter/progress_spec.rb
Refactoring ReportAdapter
Make delegation in ReportAdapter explicit
Refactor ReportAdapter some more.