Use type_traits instead of boost equivalents

Summary: Gets rid of things from boost/mpl and boost/type_traits.

Reviewed By: @oyamauchi

Differential Revision: D1564300
Fix reference and empty arguments in ReflectionFunction::__toString

Summary: When a function has no arguments, ReflectionFunction::__toString is supposed to omit the parameters section
ReflectionParameter::__toString is supposed to add the ampersand if a parameter is passed by reference

Part of #3787

Also move slow/ext_reflection/* to slow/reflection_classes
Closes https://github.com/facebook/hhvm/pull/3788

Reviewed By: @ptarjan

Differential Revision: D1566169