enable_if typedefs are not idiomatically named

[bkietz]

Names matching /is_\w+/ are conventionally reserved for bool_constants, but in cpp11 they connote a typedef of enable_if, for example (as.hpp:24):

template <typename T>
using is_integral = typename std::enable_if<
    std::is_integral<T>::value &&
        !std::is_same<typename std::decay<T>::type, bool>::value &&
        !std::is_same<typename std::decay<T>::type, char>::value,
    typename std::decay<T>::type>::type;

The prefix enable_if_ will make it clearer to newcomers that this is an SFINAE helper.

[jimhester]

Sounds like a good suggestion to me.

[bkietz]

fixed in #72