Use the location of the row for instantiated outline steps

When steps are instantiated from outline steps, they get the location
of the row (instead of the location of the outline step). To make sure
that a test case of an example row still matches the steps of the
Scenario Outline, the instantiated steps know of the outline step that
created them.
Introduce ExpandedOutlineStep for steps instanted from outlines

The ExpandedOutlineStep matches both the location of the row and the
location of the outline step.
Let the Ast::Step know how to form the backtrace line

Let the Ast::ExpandedOutlineStep to also include the outline step in
the backtrace line.
Merge PR #63 "Use the location of the row for instantiated outline steps"