This builds on mozilla#853 and adds a new language backend capable of
generating java bindings.

The generated bindings use [JNA](https://github.com/java-native-access/jna)
(I used the last version 5.13.0 while developping but this will probably
work with earlier ones as well)

Most of the new code lives in `bindgen/language_backend/java_jna.rs` and
is pretty straightfowards generation as most on the complicated things were done
in the previous refactoring.

The test suite has been updated to generate and compile the bindings for
java when running. On nice thing gained with this is the possibilibty to
put a configuration file specific to a language alongside the stardard
one and the test will take this one instead of the default one if it
exists (quite a few test use the `header` config to add some more code
inside the generated bindings, C or preprocessor code doesn't work well
in a java source file :P)

All tests are green, but that doesn't mean all the generated bindings
are correct, some cases may fail at runtime as I didn't test all
cases in a real program. I did however generate and use the bindings on a
non trivial lib I'm developping and they worked as expected.