move optional.hpp out of the way so it can't be included accidentally
chrono_io and ratio_io from Howard Hinnant
adapted for C++11 and G++ 4.7
originals at http://home.roadrunner.com/~hinnant/
error_stream<> and throw_stream(), for when printf formats are a PITA
make_backoff() free function infers duration type from parameters
define kernel::duration type from kernel::clock
use kernel::duration instead of arbitrary milliseconds for alarms, including deadlines
use chrono_io and make_backoff() in backoff test