Port to JDK20 (JEP 434 Panama second preview)

[jwharm]

The Panama APIs have changed in the JDK 20 early access release.

• Addressable and MemoryAddress have been merged into MemorySegment
• MemorySession has been removed. Memory allocation lifetimes are now handled with SegmentScope and Arena.
• Some smaller changes

I've ported the java-gi code over to the new APIs in a local branch, but will wait until the release of JDK 20 to merge it.

[jwharm]

Fixed with #61