Rework `_.keys` and `_.keys` in deps on `_.isObject`.
Rework `baseFlatten` and `_.flatten` to be shallow by default.
Rename `_.createCallback` to `_.callback`.
Remove deprecated category classifications.
Fix typo.
Keep `_.callback` even for the Underscore build because it's too complex to remove.
Remove unused dep from `_.flatten`.
Fix builds related to category changes.