Va_list support in Slinc itself

[markehammons]

As a user, I'd like to be able to write code that makes use of va_list. In Java 21, the foreign api will drop va_list support, so I'd like Slince to handle it on its own.

The changes in this pull request should help:
https://github.com/openjdk/panama-foreign/pull/763/files

The conditions for completion of this story are as follows:

• Slinc has its own VaList type totally seperate from foreign
• Java 17 has proper support for VaList, and tests pass without restriction on number of arguments
• Slinc's implementation works for Java 21
• Slinc's implementation works for Windows, linux, and Mac on Amd64 architecture