Support indexing by slicing along chunk boundaries?

[TomNicholas]

We can't support general indexing into ManifestArrays without loading data values, but we could imagine supporting slicing, so long as the slices align with chunk boundaries. The implementation would just be subsetting the underlying manifest along the sliced dimensions.

A VirtualArray like the one described in #5 could actually implement general indexing, whilst still being lazy. See the discussion on the "virtual concatenation ZEP" for details https://github.com/zarr-developers/zarr-specs/issues/288.