The select-entries lens focuses on entries of a map by key.
Unify fif and fwhen.
Extract select-keys' applicable? condition for clarity.
Support deletion of map entries.
Support deletion on each(v).
Support deletion on 'only'.
Support deletion for sets.
Using conj allows a more elegant universal functor and deletion implementation.
Now we're using conj in our functor implementation, we don't need special cases for vectors or sets.
Remove duplication in all-entries and select-entries.
State select-entries in terms of 'only'.