Support for AxisKeys

I would like to keep track of dimensions with AxisKeys through my tensorcast. Would that be possible?
```jl
let a = KeyedArray(reshape(1:6, 2, 3); time=1:2, place=1:3)
    # The ticks are \prime not single-quote.
    @cast b[time=i, time′=i′, place=j, place′=j′] := a[time=i, place=j] + a[time=i′, place=j′]
end
```