NaturalStringComparer

[StephenCleary]

Split from #23

Light-up usage of CompareTo and Equals for spans of chars to make natural string comparisons allocation-free on .NET Core 2.1+.

Light-up GetHashCode for spans of chars to make it allocation-free on .NET Core 3.0+.

[StephenCleary]

Possibly can make use of MemoryExtensions.CompareTo.