Add delegate output option

This change makes delegation more explicit than having a single output of type step_result. Now the entire output structure can be replaced with the enumeration delegate. When the spec designates outputs should be delegated, the entire output_file is read as a serialized step result.

This change all adds support for delegating through a steps type definition. Instead of providing an outputs structure to build outputs from expressions, a single key delegate will name a sub-step from which to take outputs.