Provide compatibility with MS GSL string_span<>? about gsl-lite

[mbeutel]

I'd like to introduce string_span<> when we break compatibility for version 1.0. We can introduce a compatibility macro to allow string_span, but that would affect ABI and hence trigger a warning (cf. #179).

[mbeutel]

Though apparently M-GSL maintainers aren't happy with the <> either:
isocpp/CppCoreGuidelines#1126

[mbeutel]

string_span<> was now deprecated by M-GSL due to having been removed from the C++ Core Guidelines:
microsoft/GSL#931

I wonder if we should also deprecate string_span, along with zstring_span (cf. #282, #283, #287)? Or do we keep and foster them as a gsl-lite extension?

[mbeutel]

M-GSL has since removed string_span<> entirely, cf. microsoft/GSL#1074. So I'm in favor of deprecating our string_span as well.

[mbeutel]

Following M-GSL, which deprecated and subsequently removed string_span<> and zstring_span<>, we now deprecate string_span and zstring_span as well. Deprecation messages can be suppressed by setting gsl_CONFIG_DEPRECATE_TO_LEVEL accordingly.