Support reductions over dimensions with non-Number axes

[iamed2]

C = AxisArray(collect(reshape(1:15,3,5)), Axis{:y}([:a,:b,:c]), Axis{:x}(["a","b","c","d","e"]))

# inferrable
sum(C, Axis{:y}())
sum(C, Axis{:y})
sum(C, (Axis{:y}(), Axis{:x}()))

# uninferrable
sum(C, 1)
sum(C, (1,2))
sum(C, (Axis{:y}, Axis{:x}))

The last one is due to typeof((Axis{:y}, Axis{:x})) === Tuple{UnionAll,UnionAll} which Jeff says is Not A Bug.

This should not affect any cases which already worked and definitely does not affect any cases which were tested.

[ajkeller34]

LGTM. I'll leave this open for a day in case others have comments. I noticed something related that I'll post an issue about though I don't want it to hold up your PR.