US-15: Use the _t form of type traits

[sdutoit]

N3733 US-15 says:

Given the adoption of N3655, it is possible to rephrase uses of the type traits throughout and thus both simplify and clarify the text.

It goes on to propose:

Replace each occurrence of the form “cv typename typetrait<...>::type” or the form “cv typetrait<...>::type” by ”cv typetrait_t<...>”.

[sdutoit]

Marshall Clow has indicated to me that the consensus in LWG is that this change is worthwhile.