std.traits vcs __traits

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Why is there both

http://dlang.org/phobos/std_traits.html

and the builtin

https://dlang.org/spec/traits.html

?

Should we modify std.traits to make use of __traits? I've noticed 
measurably faster compilations with __traits instead of 
std.traits for simple things such as isIntegral, isUnsigned, etc.

If so, I'll happily make that happen!