[JExtract] Generate JNI code for classes (static methods and initializers)

[madsodgaard]

This PR adds support for generating Java wrappers for Swift classes. It generates static methods and initializers.

Currently, this PR is only focused on actual allocation and initialization. Memory management is not yet addressed. We first need to provide a non-FFM version of SwiftKit and its tools for memory management, @ktoso said he would look into this.

This PR differs from the way FFM does it by delegating memory allocations to Swift instead of Java. This means we do not have to rely on any "unsafe" Java APIs for allocating memory without FFM. It also simplifies the code needed to allocate for structs, for example. We can easily change this approach later, if we decide to.

The code generates a java method init, which calls down into a native allocatingInit which does the actual allocation and returns the pointer to Java, for it to store. The plan is later to migrate over to something similiar to SwiftInstance to be used for memory cleanup.

[ktoso]

Nice, and here's the issue to track the swiftkit split-up #299

[ktoso]

Very nice so far! :-)