Fix warnings that show up with the -pedantic flag

* The `%p` format string expects a `void *`, not `int *` etc.
  Related to debug code in the tests. Fix: remove the code (9613690)

* Unattached semicolons are not allowed at the toplevel (63db360)

* `uint64` and `uint64_t` are different types
  (not flagged by `-pedantic`, but spotted while fixing the next warning) (07d2f4a)

* `"%" PRId64` expects a `long int`, not an `int64`.
  (flagged by `-Wformat`.  Not sure what's going on here: [`Int64_val` seems to return `int64_t`, not `int64`](https://github.com/ocaml/ocaml/blob/98e0051f/byterun/mlvalues.h#L268).) (dcc982e)

A couple of warnings remain:

* Implicit declaration of `alloca`: GCC (correctly) hides the definition of `alloca` when `-pedantic` is enabled.

* ISO C forbids conversion of object pointer to function pointer type.  It's a valid warning, but [libffi is designed to be used that way](https://github.com/atgreen/libffi/blob/6e8a4460/doc/libffi.texi#L670-L671).