Simplification of basic_seconds_clock.

This eliminates an unnecessary vector of pointers from the basic_seconds_clock implementation.  This may allow us to remove the vc++ hack of stopping the clock at the end of main.
This is a weird place for this enum - I missed it entirely my first time through.  Perhaps bring it up to the top?

As I said, I really don't like the idea that there's no way to tell the difference between an enumerated type, a class constant, a global variable and a method without reading the whole file, but, well, can't convince people...