Use slicing to speed up tondarray

1. Add a property to `LocalArray` maps to provide a `slice` object representing the global index space covered by that map (when possible)
2. Use that property to speed up some usages of `tondarray`.

I'm not actually using `DistArray` slicing here, I'm using ndarray slicing.  This PR should speed up `tondarray` for `Block`, `Cyclic`, and `BlockCyclic` (for `block_size == 1`)

Partially addresses #452.