slice: rework Batches and Chunks to return iterators

[creachadair]

Instead of a slice of results, return a single-valued iterator over the
selected components. A caller who wants the actual slices can use the
slices.Collect helper:

old := slice.Batches(vs, n)
new := slices.Collect(slice.Batches(vs, n))

Existing use in the target of a range do not need to change, except for
updating the target variables:

for _, c := range slice.Chunks(vs, n) {

becomes

for c := range slice.Chunks(vs, n) {