drain_range breaks invariants, causing UB #31
Description
The drain_range method makes it possible to remove arbitrary byte ranges from the underlying buffer without checking if the start or end points lie on a character boundary. This makes it possible to construct a SmallString that doesn't contain valid UTF8.
The stdlib simply panics if you attempt to drain an invalid range from a String, so I would expect it to do the same here. I'm happy to submit a PR if the maintainers would like!