Improve indexing coverage

[honno]

• Merge the current getitem indexing tests together
  • xps.indices() has come a long way in covering everything in the spec (and no more!)—I've experimented and am fairly confident that it can now supplant the current custom indexing strategies
• Test getitem indexing elements (tricky!)
• Test __setitem__
• Test boolean indexing
• Test non-set elements remain the same in __setitem__
• Test non 0d arrays in __setitem__

[honno]

I realise we also need to test boolean indexing!