This field eats a pointer for every struct timeout, and it only
exists in order to enable a set of APIs that omit the 'struct
timeouts' object. For some applications, the convenience of these
APIs is not worth the memory overhead. This patch makes it so
that applications can disable those APIs, and save some memory, by
defining TIMEOUT_DISABLE_RELATIVE_ACCESS at compile time.

remove main() testing code from timeout.c and tweak Makefile so we can compare different wheel sizes with the benchmark framework

re-add one of our old bit fill operations, as it turns out it was actually needed (probably shouldn't have worked on that code in a sleep-deprived state)

add version routines and TIMEOUT_PUBLIC prefix to exported routines so applications can optionally compile completely statically (by defining TIMEOUT_PUBLIC to `static'

add invariant assertion code, and comment our source a little better (this really needs a formal description)

use timeouts_ prefix instead of timer_, which is in the POSIX namespace (they get all the good prefixes!

work backwards as well as forwards when calculating mask to overcome cummulative error when step is greater than period