id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
777153550,MDExOlB1bGxSZXF1ZXN0NTQ3NTIwNjI0,4746,Faster unstacking,5635139,closed,0,,,22,2020-12-31T22:55:26Z,2021-01-24T23:48:09Z,2021-01-24T23:48:05Z,MEMBER,,0,pydata/xarray/pulls/4746,"<!-- Feel free to remove check-list items aren't relevant to your change -->

`asv profile unstacking.Unstacking.time_unstack_slow` runs in 10ms now, down from 1076ms.

Todos:
- [x] Resolve how to handle sparse arrays — they currently go through the existing functions, and we have one test failure
- [x] Decide what to do with the existing functions — in particular `Variable.unstack` wouldn't work with the new code, since it requires the index which is being unstacked, and variables don't have indexes. The existing function will fail on any missing indexes, but may still be useful.
- [x] Clean up the code a bit — remove `_fast` from names where possible / remove some dead comments
- [x] Passes `isort . && black . && mypy . && flake8`
- [x] User visible changes (including notable bug fixes) are documented in `whats-new.rst`
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396084551,MDExOlB1bGxSZXF1ZXN0MjQyMzg1OTg0,2650,rolling_exp (nee ewm),5635139,closed,0,,,22,2019-01-04T22:10:07Z,2019-06-24T15:26:58Z,2019-06-24T15:20:32Z,MEMBER,,0,pydata/xarray/pulls/2650,"- [x] Duplicate groupby / rolling interface
- [x] Integrate with apply_ufunc, including orientation and coords
- [x] Define com / span / alpha interface
- [x] Implement for Variable & Dataset
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