Added test that detects if clock_cast<Dest>(Source) properly
SFINAEs if clock's are not castable, this includes test for
steady_clock that is not castable to any wall-clock.
Secondly added steady_based_clock based on steady_clock
(as name indicates), that clock_cast may be extended to clock
non-related to wall-time (sys/utc) using conversion traits.
Final example is pair of ambiguous clocks (amb1/amb2_clock) that can convert
to each other either using sys_clock or utc_clock.
Then the conversion from amb2 to amb1 is disambiguated
via trait specialization.
