[x] `swap_delete`
[x] `n_chunks` / split_n
[x] `sized_chunks`
[x] deprecate `length` for `size`
[-] `reduce`
[x] deprecate first/last rest/leading
[x] add at? (get but with negative indices)
[ ] the append/lens optimisation
[ ] measure when O(log n) concat is worth it vs not, heuristics etc
[x] deprecate iv.range
