improve getall/setall for staticarrays

[aplavin]

Now, it's more consistent:

• getall comptime-known length: Tuple
• getall runtime length: Vector

This change lets them nicely propagate and infer for both get- and setall.

Non-breaking: container types were specifically allowed to change in getall docsting.

[aplavin]

@jw3126 would really appreciate a look/review here!
I find this code basically required for any nontrivial getall/setall use with staticarrays. Would you please check that everything looks fine?

[jw3126]

LGTM