I've decided that I want reserve to mean only allocate backing memory,
not modify the size of the string, but I didn't want to rework much code
in the process. I might eventually get right of the open functions, but
I wouldn't be surprised if that's another decade or two in the future.
Just head and tail are atomic, but it seems to work nicely (at least on
intel). I actually had more trouble with gcc (due to accidentally
testing lock-free with the wrong ring buffer... oops, but yup, gcc will
happily optimize your loop to spin really really fast). Also served as a
nice test for C11 threading.
