Improve performance when decoding hex.

[brian-reichle]

Only affects .Net Framework and .Net Core 8.0 as later versions of .Net Core will use the vectorized implementation provided by the runtime.

BenchmarkDotNet v0.15.8, Windows 11 (10.0.26200.7840/25H2/2025Update/HudsonValley2)
Intel Core i7-9750H CPU 2.60GHz (Max: 2.59GHz), 1 CPU, 12 logical and 6 physical cores
.NET SDK 10.0.103
  [Host]             : .NET 10.0.3 (10.0.3, 10.0.326.7603), X64 RyuJIT x86-64-v3
  .NET 8.0           : .NET 8.0.24 (8.0.24, 8.0.2426.7010), X64 RyuJIT x86-64-v3
  .NET Framework 4.8 : .NET Framework 4.8.1 (4.8.9221.0), X64 RyuJIT VectorSize=256

Method	Job	HexValue	Mean	Error	StdDev	Ratio	Allocated	Alloc Ratio
FromHexString	.NET 8.0	0123456789ABCDEF	31.233 ns	0.5229 ns	0.4636 ns	0.82	-	NA
OldFromHexString	.NET 8.0	0123456789ABCDEF	38.117 ns	0.7843 ns	1.9240 ns	1.00	-	NA
FromHexString	.NET Framework 4.8	0123456789ABCDEF	36.025 ns	0.6336 ns	0.5927 ns	0.54	-	NA
OldFromHexString	.NET Framework 4.8	0123456789ABCDEF	67.143 ns	0.8689 ns	0.7703 ns	1.00	-	NA