feat: add benchmark to measure GetHash

[virzak]

I noticed that GetHash still uses an object, so tried to add a benchmark and the improvement is significant:

// * Summary *

BenchmarkDotNet=v0.13.4, OS=Windows 11 (10.0.22621.1105)
AMD Ryzen 9 5900HS with Radeon Graphics, 1 CPU, 16 logical and 8 physical cores
.NET SDK=7.0.102
[Host] : .NET 7.0.2 (7.0.222.60605), X64 RyuJIT AVX2
Job-DRNVHW : .NET 7.0.2 (7.0.222.60605), X64 RyuJIT AVX2

Runtime=.NET 7.0 Toolchain=net7.0 IterationCount=3
LaunchCount=1 WarmupCount=3

Method	Mean	Error	StdDev	Ratio	RatioSD	Gen0	Allocated	Alloc Ratio
Struct	11.511 ns	7.585 ns	0.4157 ns	0.43	0.04	0.0029	24 B	NA
WithLayout	10.225 ns	5.764 ns	0.3160 ns	0.38	0.02	0.0029	24 B	NA
WithLayoutNoBoxing	4.158 ns	2.832 ns	0.1552 ns	0.16	0.02	-	-	NA
Class	26.755 ns	37.681 ns	2.0654 ns	1.00	0.00	-	-	NA
ClassLayout	25.655 ns	33.121 ns	1.8155 ns	0.97	0.14	-	-	NA