Peformance enhancement: replace SizedStack with TailBuffer

Currently there are two comment stripping readers (XML and JS)
They use the SizedStack to maintain a lookbehind buffer. This creates a number of performance issues:
1. It creates an object for every character that is read. This creates millions of short lived objects. 
2. The SizedStack inherits from Vetcor that has synchronized methods and is not performant
3. When extracting the string from the stask a new String object is created for each Character. More short lived objects.

The TailBuffer gets round this by storing chars in a  fixed length native char buffer