Add regular std::span support to spanbufs

[malachib]

In particular, accepting std::span as inputs on constructor. Likely behind the scenes this gets converted to and estd::span.

Future code may consider aliasing std:;span to estd::span and/or doing an Impl pattern on the spanbuf to switch to std::span or estd::span specifically. Perhaps take a stab at some of that in this issue.