Preemptive refactoring of result_set_presenter

We intend the plain array format to be removed at some point.
I want to add more things to the hash format (specifically spelling
suggestions), so it feels wrong to call it ``present_with_result``. Let's
instead have one well-named method and treat the undesirable case as the
exception.
Harmonise API with the other presenter
Tweak the API of Suggester

Return an array, even though we only currently generate one suggestion.
This should make adding additional suggestions easy later on.

Make the method name explicit about what type it returns.
Return spelling suggestions for search terms