[jextract] Generate code for free functions with primitive types in JNI mode

[madsodgaard]

This PR adds support for a new mode for jextract that would generate JNI code instead.

We will start of with generating Java bindings and Swift thunks for free functions and primitive types.

[madsodgaard]

Should probably safeguard the same way FFM does, by just naming them _0, _1 etc.. however, I am not sure when parameterName actually can be nil? 🤔

[rintaro]

func foo(_: Int) case should be nil. Not so common, but for example:

protocol SomeProtocol {
  init()
}
func foo<T: SomeProtocol>(_: T.Type) -> T {
  return T.init()
}

[ktoso]

yeah we should handle the nil, it's not very common but we should handle it.

We can maybe centralize it somehow to provide some one place where we do effectiveJavaParameters()? The impl of naming those could be just:

[...].enumerated().map { idx, param in
  "\(param.type.javaType) \(param.paremeterName ?? "arg\(idx)")"
}

At the same time maybe it's a good time to then fix them to become maybe arg0 which is a common Java way to spell those "when names are unknown" 🤔

[ktoso]

Good first step -- consider adding a test as well so we can keep the generated signatures tested using unit tests.

I'd be happy to just merge these small steps one by one but let's add a small test here maybe? 👍

[madsodgaard]

The last thing I need to handle in this PR, is the unsigned integers which do not conform to JavaValue. I guess we should just not support generating code for them, for now?

[ktoso]

Yeah totally ok to do that separately 👍 Please file an issue about it so we don't forget :)

[ktoso]

LGTM :-) Great first step and thanks for adding the tests!