improve const correctness of disk I/O code

[arvidn]

by making readwrite() a template that can work with both span and span.

[glassez]

Isn't span<char const> constructible from span<char>?

[arvidn]

I believe it is. the issue here is that it's being passed to a function, and that function then passes it back into a callback. So it's being "laundered" by the readwrite() function signature. By making it a template, it can either char const or char and the callback takes the same type

[glassez]

the issue here is that it's being passed to a function, and that function then passes it back into a callback.

Got it.