[v3] ext/type_traits.h

[Benjamin Kosnik]

This is a preliminary attempt at removing duplicate traits/type_utils.
I think this is a good idea, although I am unsure if this should be
ext/type_traits, ext/type_utils, or something else. Peter makes the
argument that these are type utilities for metaprogramming, mostly, so
should reflect that. Seems reasonable.

Similarly for many of the other names, suggestions welcome.

I sure would like to move std::__enable_if in here too. 

For the hastable fixups for extract1st, this currently uses
std::_Select1st. However, I think it should be possible to use
std::tr1::get<0, _Pair>, which should do what needs to be done.
(Right?). However, it looks like we'd have to add specializations to
__get_pair and overloads for std::tr1::get, so that we could use _Pair
as a template argument, and not construct an instance of std::pair out
of a pair of template arguments. Whew. Thoughts?

tested x86/linux

-benjamin
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