Simplify initial index labels PR

* removed support for nested traversal of `index_labels`. Support for
  this can be added later but it required making a lot of decisions
  about managing labels that up front that would be better addressed
  through iterative PRs
* removed `has_index_labels`. There are some odd corner cases for this
  one. Particularly for `SubArrays` where the presence of labels in the
  parent don't always have a clear way of propagating forward. Again,
  we can address this one but it will take some decisions about how
  labels are propagated.
* `UnlabelledIndices` and `LabelledIndices` are types that provide a
  more clear structure to what a label is and how they are accessed. For
  now access only is supported through `getlabels` and `setlabels`

Author: Tokazama

src/ArrayInterface.jl

@@ -7,7 +7,7 @@ import ArrayInterfaceCore: allowed_getindex, allowed_setindex!, aos_to_soa, buff
     safevec, zeromatrix, ColoringAlgorithm, fast_scalar_indexing, parameterless_type,
     ndims_index, ndims_shape, is_splat_index, is_forwarding_wrapper, IndicesInfo, childdims,
     parentdims, map_tuple_type, flatten_tuples, GetIndex, SetIndex!, IndexLabel,
-    LabelledIndices, getlabels, UnlabelledIndices, defines_strides, stride_preserving_index
+    LabelledIndices, getlabels, setlabels!, UnlabelledIndices, defines_strides, stride_preserving_index
 
 # ArrayIndex subtypes and methods
 import ArrayInterfaceCore: ArrayIndex, MatrixIndex, VectorIndex, BidiagonalIndex, TridiagonalIndex