Double the speed of DecimalFormat creation.

Our calls to unum_setSymbol were making us O(n^2); switching to the C++ API
and doing a bulk update is a huge win. (ICU is really a C++ library with a
C wrapper. It's always going to be slightly wasteful to go via C, but here
it's especially harmful.)

The new ScopedJavaUnicodeString provides a best-of-breed bridge between Java
strings on the Java heap and the UnicodeString type that ICU wants. I'll come
back and switch more of our ICU JNI over in a later patch.
3 files changed