timers: add timers interface

Mimic SBCL API: http://sbcl.org/manual/index.html#Timers

setitimer and getitimer for POSIX doesn't require threads, but if we want to have it on Windows, we'll have to create a separate thread for interruptions. That may be also useful for builds on POSIX if the programmer doesn't want to lose SIGALRM.

This feature will be useful for implementing scheduler for green threads (will be easily composable there).