fix k-fold-cv helper functions

[avehtari]

As mentioned in #49 (comment)

• Make our kfold helper functions to match with https://scikit-learn.org/stable/modules/cross_validation.html
• Allow non-binary discrete x for kfold_split_stratified

[jgabry]

I'm glad you're making all these PRs! I think there are a few tiny things to change but otherwise this looks good.

The tests in tests/test_kfold_helpers.R also need to be fixed and the NEWS file updated, but I can take care of those things if you want.

[avehtari]

Sorry for making a pull request with broken code. I modified tests, too, as I need to learn to use tests.