ffi: java interop improvements

stdlib/src/main/pork/ffi/malloc.pork

@@ -1,5 +1,8 @@
 export func malloc(size)
-  native ffi "c:malloc:void*"
+  native ffi "c:malloc:void*:size_t"
+
+export func calloc(size, count)
+  native ffi "c:calloc:void*:size_t,size_t"
 
 export func free(pointer)
-  native ffi "c:free:void"
+  native ffi "c:free:void:void*"