Migrate away from JNA to raw JNI

[DavyLandman]

We want to move to JNI for performance reasons, and it also clarifies the confusion that the dual license situation of JNA might create. We're using it as apache 2, but not all tools (including maven meta-data) support dual-licenses and this could cause down-stream confusions which we rather avoid.

For JNI we will need to setup cross compilation, some places where this is discussed:

• https://bsideup.github.io/posts/jni-cross-compilation/
• https://github.com/luben/zstd-jni/tree/master/.github/actions/qemu-cross

We'll only need to add support for JNI for macOS, and then mainly for x64 and arm64.

[DavyLandman]

Reading a bit more, it might be interesting to see if we can use https://github.com/jnr/jnr-ffi which has a more clear license scenario and is also faster.

Edit: sadly it's not suited, it's not been getting new commits, and for example doesn't work of aarch64 nor can it link to CoreServices.framework, it always looks for dylib.

[DavyLandman]

Fixed in #61