Generalize MacExpr / MacPat / MacItems

[kmcallister]

There should be a single type implementing MacResult with Option fields for all the types MacResult can return, and a Default impl that fills in None.

[gamazeps]

On it :)

[eddyb]

IMO this is the wrong way to go about doing things, a macro that can expand to different nodes depending on context would need to produce all of the possible expansions ahead of time.

[kmcallister]

@eddyb: No, you can still implement MacResult yourself; this is much more convenient in the common case where it's no problem to produce all of the expansions ahead of time.