provide multi_array::swap

... or at least make its members protected to allow a subclass to do so.

`swap` would be substantially the same as the current `resize` method.

Background: I use a subclass of `multi_array` in order to resize during assignment to get around [the most dagnerous multi_array pitfall](http://coliru.stacked-crooked.com/a/53484c30d4cbc065). This works, but I'd also like to fix another longstanding inefficiency ( #24 ) while I'm at it. Unfortunately the `private` data members make it impossible to fix in my subclass (barring a type-punning hack).