ffi: support for java native functions

examples/java.pork

@@ -0,0 +1,10 @@
+func java_system_err()
+  native java "java.lang.System:static-getter:err:java.io.PrintStream"
+
+func print_stream_println(stream, line)
+  native java "java.io.PrintStream:virtual:println:void:java.lang.String"
+
+export func main() {
+  let error = java_system_err()
+  print_stream_println(error, "Hello World")
+}