I've found that frequently a void function and a pointer are either
more or less than we want to attach to each timeout object.  This
patch makes it so that the user can define TIMEOUT_CB_OVERRIDE to
indicate that they have included their own definition of struct
timeout_cb.

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