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.
What's with the weird indentation?  You could easily fit the whole thing in one line, and it wouldn't be a hard read.  

But that did point out that you use this subexpression twice, it might be a good candidate for a little function...!