span index is an int instead of unsigned int

[rianquinn]

Is there a reason why span takes an int instead of unsigned int? It's really annoying when you have to convert index's to unsigned int every time you want to index into a span (assuming your using -Wsign-conversion -Wconversion).

What's even more annoying is how it's not consistent with std::vector and friends which take unsigned ints, so your constantly having to static_cast / verify sizes via numeric_limit<>.

[neilmacintosh]

This issue has been raised and answered before in #322, #238 and #171, so I'll close it as a duplicate.