Upgrade incompatibilities: notes for developers

From previous versions to 2.0.0.0

No compatibility is ensured. Most APIs haven't changed, but no guarantee is offered.
The most important change is probably the disparition of struct tai and struct taia, replaced with the tai_t and tain_t types. Attosecond precision has been removed - processor speed is almost capped now, and it looks like nanosecond precision will be enough for the foreseeable future.
Programs should now link with -lskarnet followed by the appropriate sysdeps links.