Use view/span

[billyquith]

We already have ponder::string_view. We have local copy because of inconsistent support in C++ libs.

std::array_view seems to have been renamed as std::span. What happened to array_view?

• https://github.com/martinmoene/gsl-lite
• Simple array_view implementation: https://github.com/mclow/snippets/blob/master/array_view.cpp