When replace is passed a string whose length is greater than end - start, it would cause the drain on the line to panic as the range start was bigger than the range end. To address this, I switched from using drain and adding chars to using splice, as it does what the code intended.

While looking into this, I also realized the true cause was that we were trying to trim the middle of spans that were too short to trim, and decided to fix that as well.

Fixes #391