labels and valkeys does not work on Axis

I'm not sure if there is anything preventing this, but it would sometimes be convenient to be able to ask about the labels in an axis, similar to how it can be done for a ComponentArray.

julia> uax
Axis(u1 = 1, u2 = 2)

julia> labels(uax)
""

julia> valkeys(uax)
ERROR: MethodError: no method matching valkeys(::Axis{(u1 = 1, u2 = 2)})
Closest candidates are:
  valkeys(::ComponentVector{T, A, Axes} where {T, A, Axes}) at /home/fredrikb/.julia/packages/ComponentArrays/Xo6aP/src/componentarray.jl:313
Stacktrace:
 [1] top-level scope
   @ REPL[27]:1

If I try to indx into an Axis with integers, I get something, but I can't judge if it makes sense or not.

julia> uax[1]
ComponentIndex(1, NullAxis())

julia> uax[2]
ComponentIndex(2, NullAxis())

julia> uax[1:2]
ComponentIndex(1:2, FlatAxis())

I'm trying to implement indexing with symbols (https://github.com/jonniedie/ComponentArrays.jl/issues/88) and this issue came up.

Reproduce the Axis, labels, valkeys, and integer-indexing examples from the issue, then compare them with ComponentArray behavior in componentarray.jl around line 313. Review the related symbol-indexing issue #88 and establish the expected Axis behavior before adding tests that demonstrate the completed interface.