Replace StringBuffer with StringBuilder where possible, add to forbidden-apis

This is pretty minor, but I found a few issues with the toString implementations while looking through the facet data structures.

The most egregious is the use of string concatenation in the IntArray class. I have fixed that using StringBuilders. I also noticed that other classes were using StringBuffer instead of StringBuilder. According to the javadoc,

"This class is designed for use as a drop-in replacement for StringBuffer in places where the string buffer was being used by a single thread (as is generally the case). Where possible, it is recommended that this class be used in preference to StringBuffer as it will be faster under most implementations."