Use FastPrintWriter... everywhere.

One problem this turned up is, because FastPrintWriter does
its own buffering, a lot of code that used to use PrintWriter
would fail -- if it pointed to a StringWriter, there was no
buffering, so it could just immediately get the result.  Now
you need to first flush the FastPrintWriter.

Also added some new constructors to specify the size of buffer
that FastPrintWriter should use.

Change-Id: If48cd28d7be0b6b3278bbb69a8357e6ce88cf54a