Add ReadOnlySpan<char> overloads to the DecimalNotationFormatter

[lipchev]

In order to easily support the ISpanFormattable interface, see about adding an overload to the DecimalNotationFormatter that works with a ReadOnlySpan<char>

This should further reduce the allocations required for formatting strings that contain a Fraction as part of the format.

This is up for grabs (I'll probably look into it at some point):

[danm-de]

Hey,

I have nothing against the introduction of Span/ReadOnlySpan method overloads. Several people from my work environment pointed out to me, that the use of #if/#else preprocessor directives makes the code confusing.

I may outsource the affected functions to different files in future. Example file layout:

• Fraction.FromString.cs (general, available in both, netstandard and netcore)
• Fraction.FromString-netstandard.cs (.NET Standard implementation)
• Fraction.FromString-netcore.cs (.NET Core implementation)