functional, split tr1 and c++0x impl's ?

[Benjamin Kosnik]

> As soon as you have evidence than ~10 ifdefs are required just split,
> split, split. This tr1_impl idea, suggested by Doug, is very nice,
> IMO, because it allows for a smooth transition to the C++0x
> specifications, but as soon as we have evidence that C++0x is
> appreciably different, let's just split and obtain something much
> more clean even if a little redundant.

Agreed. 

-benjamin