Add Span API microbenchmarks for slow span to allow comparison with fast span

[ahsonkhan]

Related to issue #7442

https://github.com/dotnet/coreclr/tree/master/tests/src/JIT/Performance/CodeQuality/Span

[gkhanna79]

@ahsonkhan Who should this be assigned to?

[ahsonkhan]

I can assign it to myself once I have capacity to work on this. I was thinking of leaving it as up-for-grabs in the meantime.

[stephentoub]

We're not investing in "slow span". Closing.