Feature request: Support for ReadOnlySpan<char>

[Marvin-Brouwer]

I currently have a code base where I pass around a string as a ReadOnlySpan<char> and I'd like to call ToString() at the latest moment possible. However, I'd also like to guard against NullOrEmpty or NullOrWhiteSpace. Perhaps even against a pattern but I suppose that's impossible with regex and span.

[KonH]

@Marvin-Brouwer please take a look at the pull request above - I tried to implement what you looking for.
I'm not happy with semantics - it doesn't make sense to actually check span against null.
But it is much more friendly for an end user to have same name overloads and not think about the underlying type.

[Marvin-Brouwer]

@KonH I made a review on your changes.
I have some suggestions