Emit warning when user passes string instead of list to OneOf

I just got caught.

I wrote

```py
validate=validate.OneOf("one", "two")
```

instead of

```py
validate=validate.OneOf(["one", "two"])
```

`"one"` would pass because `"one" in "one"` is `True`.

It might save users trouble if we emitted a warning when a string is passed instead of a non-string iterable.

In practice, it only affects lists made of two elements (or one but users would use `Equal`), so no big deal.

Start at the OneOf validator entry point and trace how its choices are accepted. Reproduce the string-versus-list case, then add coverage for the warning and confirm the existing list form remains valid; done means the misuse is warned about without changing normal iterable validation.

Written by the indexing model from the issue text.