jextract closure support should not specialize () -> () and () -> T shapes, and just use Runnable and Supplier

[ktoso]

That's idiomatic Java types to use for these situations and we can save ourselves some codegen by doing so.

So when we see

func test(a: () -> ()) 

We should make that Runnable rather than generate an one-off type for this.

The same for Supplier probably