Reshape codec #4541

Describe the new feature you'd like

I implemented the reshape codec in a separate package but it's not yet on PyPI. In clbarnes/zarr-python-reshape#1 , @normanrz suggested incorporating it into mainline zarr-python; I agree that it will be important as it will underpin the use of most image codecs.

The implementation is small and pretty simple, and has needs no additional dependencies; it lives in one 300-line file.

I can raise a PR if it would be accepted.