Add implementation for ISpanFormattable

[vyacheslav-volkov]

First of all thanks for this project, the best implementation on the market.
I think it would be a good idea to implement the ISpanFormattable interface, this will reduce allocations in some cases.

[vyacheslav-volkov]

Also you can optimize this code if BigDecimal implements ISpanFormattable:

 return float.Parse(value.ToString(ToDecimalOrFloatFormat), ToDecimalOrFloatStyle, CultureInfo.InvariantCulture);
 return double.Parse(value.ToString(ToDecimalOrFloatFormat), ToDecimalOrFloatStyle, CultureInfo.InvariantCulture);
 ...

[mikernet]

Yeah, this would definitely be useful, but the string formatting would need a bit of a complex refactoring to support that so I haven't had time to do it. The string formatting is all a bit obtuse right now, mostly because the runtime BCL hides a lot of the functionality needed to implement it easily for a custom numeric type.