[jnigen] Few improvements in generated C code including exception support. (https://github.com/dart-lang/jnigen/issues/87)

* C bindings no longer use fully qualified names. Instead, it uses number-renamed simple names. The classes are sorted in the summarizer to ensure the order is deterministic. This enables shorter symbol names and less verbose bindings.

* C bindings are formatted using same style as Dart SDK, using clang-format. If clang-format is not available, jnigen will issue a warning.
Closes: https://github.com/dart-lang/jnigen/issues/84

* C bindings return `JniResult`. The older `Jni.checkException` stopgap is removed. It reduces one line per function binding in dart. Lastly exceptions will work properly on android
Closes: https://github.com/dart-lang/jnigen/issues/56