Add descriptors for std::tuple

[EvanKirshenbaum]

[imported from HPE issue 180]

There's no good reason std::tuple shouldn't be GC-friendly in the same way std::pair is, but I couldn't find a way to get an offset for the element that the compiler would treat as a compile-time constant expression. (The tricks I used for superclasses and fields didn't work.)

I left my latest attempts in, guarded by #ifdef GC_FRIENDLY_TUPLES