type_traits migration

[SergioRAgostinho]

There are a number of type_traits exposed in the standard. I'm interested in figuring out how much of mpl, can we remove.

Valuable (perhaps outdated) references:

• http://pdimov.com/cpp2/simple_cxx11_metaprogramming.html
• http://pdimov.com/cpp2/simple_cxx11_metaprogramming_2.html