The Matcher class has a few public methods that take StringBuffers,
and we've found it useful to add similar versions that take StringBuilders.

It has two benefits:

- Users don't have to convert from one to the other when they want to use
the method in question. The symmetry is nice.

- The StringBuilder variants are faster (if lock optimizations don't kick
in, which happens in the interpreter and the client compiler). For
interpreted / client-compiled code, we saw something like a 25% speedup on
String.replaceAll(), which calls into this code.