provide type-inferred length

[bgctw]

A maybe-nice feature request.

Using StaticArrays.jl often helps avoiding allocation in bottle-neck code. When creating the StaticArray, the length or size must be known to the compiler.
ComponentArrays have this information in its type in its axes. However, currently length and size are based on the underlying array, for which the size might not be known at compile time.

I suggest implementing Base.length for an Axis and use this to implement Base.length and Base.size on getaxes(cv).