timer.h takes the place of the old time.h, while time.h itself has been pared down to only non-async-related stuff.

Since async-io.h includes timer.h, I suspect no one will be broken by this change.

I could have instead moved the non-async parts of time.h into a new header to avoid any possibility of breakage, but time.h is the correct name for the core header, and timer.h is the correct name for the async parts (the Timer class)... so I'm risking it.