Depessimize string conversions.

Depessimize string conversions.

Why does this idiom persist? It's ugly, and it's the least efficient way to do
it. (I found the ones in DecimalFormatSymbols while invesigating why
"new SimpleDateFormat()" burns through so many StringBuilders. grep(1) found
the rest.)

The DocumentBuilderImpl removes an unnecessary level of indirection, since we
implement Character.toString in terms of String.valueOf. (I wouldn't have
bothered except this was the only use of Character.toString in the core
libraries, and I added it myself a few weeks ago.)