Implement API of nback.llvm using LLVM C API via JNI

[jclark]

When we do #17, it would also be good if we use an alternative module to nback.llvm that calls the LLVM C API via JNI.

We can use this to do it:

https://github.com/bytedeco/javacpp-presets/tree/master/llvm

This will validate that when we self-host we will be able to easily switch to the C API.