clang -Wunused-function warns about a 'static constexpr' function
defined in a header if the header is ever included in a file which does
not call the function. Move the function into class scope to silence
the warning and prevent the compiler from considering it when resolving
operator*(T1,T2).
Reported-by: btb <https://github.com/dxx-rebirth/dxx-rebirth/pull/54#issuecomment-86252750>