Override ipython repr methods.

[dcherian]

Closes #1716

This avoids expensive lookups against object stores.

TODO:

• Add unit tests and/or doctests in docstrings
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[MSanKeys963]

Thanks, @dcherian.
I've launched the workflows.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.99%. Comparing base (2534413) to head (adaff54).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1724   +/-   ##
=======================================
  Coverage   99.99%   99.99%           
=======================================
  Files          38       38           
  Lines       14580    14611   +31     
=======================================
+ Hits        14579    14610   +31     
  Misses          1        1           

Files	Coverage Δ
zarr/hierarchy.py	100.00% <100.00%> (ø)
zarr/tests/test_hierarchy.py	100.00% <100.00%> (ø)

[jhamman]

Thanks @dcherian!