declares all repeat<> friends of all Repeater<>
declares all repeat<> friends of all Repeater<>

There's a gcc bug that causes it to *very explicitly* not allow
declaring a constexpr friend specialization which is fixed in trunk as
of right now, and will probably get released with gcc-5.1.1
http://stackoverflow.com/questions/32174186/

ryanhaining/cppitertools,jkhoogland/cppitertools,jkhoogland/cppitertools,jkhoogland/cppitertools,ryanhaining/cppitertools,ryanhaining/cppitertools