SwiftArena type to support managing lifetimes of Swift objects from Java

[ktoso]

This is an idea to improve the safety of Swift objects created from java.

Something like:

try (var arena = SwiftArena.ofConfined()) {
  var obj = new MySwiftObject(arena)
  hello(obj)
} // count down obj, assert that destroyed
// if obj refcount > 1, it was "leaked" 
// beyond lifetime of the arena which is a programming error

We can have the arena perform the actual allocation or still leave it to the underlying init, how we should look at the refcounts and make sure we don't "leak" such managed object.

[ktoso]

Initial work done in #67

We'll keep polishing those, and there's some platform dependent issues still that have bugs logged.