Add ability to include smaller last batch

[nkoppel]

This is a breaking change that changes the name of batch to batch_exact, and redefines batch to include the smaller last batch if it exists. As documented, the last batch will always contain at least one element.

[coreylowman]

Thoughts on making this non breaking in the following way:

1. Keep existing batch method, but add #[deprecated] mentioning the new batch_exact
2. Add batch_exact like you do so here
3. Add batch_with_last (what you have here in the PR as batch). Alternative names might be batch_uneven, batch_unexact, batch_keep_last

I'd like to start moving towards using deprecation more and this could be a good place to try it out.

[nkoppel]

These changes should now be completely non-breaking.

[coreylowman]

Nice changes! 👍