az_span for literal strings

[PTaylor-us]

Is your feature request related to a problem? Please describe.
Casting away const is not ok.

Describe the solution you'd like
At one time in this library there was both an az_span (immutable) type and an az_mut_span (mutable) type. For some reason this was done away with and the az_span type was made mutable. This can be a big problem when an az_span is created from a literal string or some other const string.