Modernize syntax of enable_if and traits to use _t helpers

Modernize syntax of enable_if and traits to use _t helpers

From c++ 14, "typename std::enable_if<something>::type"
can be reduced to
              "std::enable_if_t<something>"

as can traits with ::type

Note that when we switch to C++ 17, a number of traits with
::value can be similarly shorthanded with _v

Change-Id: I92d4502c51ac613fa81f130ed3127c812c4e0a6f