If std::common_type<range::index_type>... finds a common index_type
among all the zipped ranges, pass that common type through as
zip<...>::index_type. Otherwise, set zip<...>::index_type to void.
This allows enumerate(zip(...))) to report a better index_type.
std::find_if needs common iterator traits. Add the relevant type
definitions to zip_iterator.
Also add them to d_range, to avoid errors when a range is zipped.
This is necessarily incomplete since it can only check ranges that have
a compile-time static size. However, it catches some simple mistakes,
and imposes no runtime cost, so it is still useful.
The minimum supported compiler versions now provide a depth-efficient
implementation of std::make_index_sequence, which removes the last
reason to carry a private implementation. In the case of clang, it
appears to have a special compiler intrinsic used to implement its
std::make_index_sequence.
Switch to the compiler-provided version for both gcc and clang.
