Combine std::tuple/std::pair logic

The std::pair caster can be written as a special case of the std::tuple
caster; this combines them via a base `tuple_caster` class (which is
essentially identical to the previous std::tuple caster).

This also removes the special empty tuple base case: returning an empty
tuple is relatively rare, and the base case still works perfectly well
even when the tuple types is an empty list.