C# Levenshtein improvements (~35%)

[antrv]

Description of changes

Accessing ReadOnlySpan and Span in C# always involves boundary checks. To improve the performance, references and reference arithmetic can be used instead.

My changes improve the performance approximately by 35% on my AMD CPU.

The benchmarks are done using BenchmarkDotNet library.

BenchmarkDotNet v0.15.2, Windows 11 (10.0.26100.4652/24H2/2024Update/HudsonValley)
Unknown processor
.NET SDK 9.0.302
  [Host]     : .NET 9.0.7 (9.0.725.31616), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI
  DefaultJob : .NET 9.0.7 (9.0.725.31616), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI


| Method  | Mean     | Error   | StdDev   | Median   |
|-------- |---------:|--------:|---------:|---------:|
| OldCode | 156.3 ms | 6.23 ms | 17.87 ms | 149.9 ms |
| NewCode | 109.1 ms | 4.03 ms | 11.89 ms | 106.6 ms |