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.
When partial_range is used on a container that defines index_type,
define that same index_type on the partial_range. This allows other
callers, such as enumerate(), to maintain the correct index type.
Prior to this, an xrange always started at the begin term and
incremented by 1 per step until it reached the end term. There was no
support for a step size other than 1. Add support for custom step size.
It is the caller's responsibility to pick a step size that will
eventually lead to (iter != end) evaluating to false.
Switch valptridx error style dispatching from using macro pasting to
using C++11 user-defined literals. This makes the code a bit easier to
read, and removes the need for a C99-conforming preprocessor here, which
should help anyone trying to port to Microsoft Visual Studio.
The new implementation also fixes a limitation of the previous
implementation. Before, an override that referenced an invalid name
could be silently ignored. Now, incorrect overrides cause an attempt to
use an undefined instantiation, which fails with a compilation error.
