Use suggestion applicabilities everywhere

[Manishearth]

rust lets you tag suggestions with an applicability level, so that tools like rustfix know which suggestions are applicable.

We should use this here too. span_to_snippet calls should generate HasPlaceholders applicabilities during the fallback, and if macros are involved we should use MaybeIncorrect, perhaps (or add a InMacro applicability).

Currently I don't want to advertise rustfix too much since the UI is edition focused, but once that settles we should be prepared to have rustfix work well with clippy.