ewma: ensure 64-bit alignment of uncounted

This bug was already fixed by #20 (and reported in #19) but was rolled
back after merging #17.

`uncounted` has been moved to top with a comment explaining that it
should stay at top. This is not mandatory but this is the easiest way to
ensure a good alignment in the future without taking too much care.

This alignment is required to use atomic functions as explained here:
 http://godoc.org/sync/atomic#pkg-note-bug