Vectorized SequenceCompareTo for Span<char> (#17237)

Vectorized SequenceCompareTo for Span<char> (#17237)

- This change makes the compare for very short Span strings a bit slower and for longer Span strings many times faster.
- Switch several places where it was a clear benefit to use it.
`String.CompareOrdinal(string,string)` is notable exception that I have left intact for now. It is fine tuned for current string layout, and replacing with a call of vectorized SequenceCompareTo gives mixed results.