Improved switch wasmtime::Span vs std::span

[redradist]

No description provided.

[alexcrichton]

I'm personally historically wary of patterns like this because it seems like you can create subtle dependencies one one mode or another. Do you know if that's possible here, or is this a common pattern in C++?

[redradist]

@alexcrichton It is common pattern if we do not have currently std::span
After yesterday implementation I though little-bit and found case where implicit conversion did not work:

auto span = call_function(); // It will be wasmtime::Span but user want to use full `std::span` with all available API

With template alias if user has C++20 then it automatically will have span class with all available api
wasmtime::Span compatible with the subset API of std::span

[alexcrichton]

Ok seems reasonable to me, thanks again!