Improved performance of the Base64Decoder
Improved performance of the Base64Decoder

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.22000
Intel Core i7-9700 CPU 3.00GHz, 1 CPU, 8 logical and 8 physical cores
.NET SDK=6.0.300-preview.22154.4
  [Host]     : .NET 6.0.3 (6.0.322.12309), X64 RyuJIT
  DefaultJob : .NET 6.0.3 (6.0.322.12309), X64 RyuJIT


|           Method |     Mean |   Error |   StdDev |   Median |
|----------------- |---------:|--------:|---------:|---------:|
| OldBase64Decoder | 414.5 us | 8.20 us | 11.76 us | 413.9 us |
| NewBase64Decoder | 357.6 us | 6.56 us | 12.48 us | 351.8 us |

jstedfast/MimeKit,jstedfast/MimeKit,jstedfast/MimeKit