Adding Span API perf tests for comparison with arrays and to monitor progress (dotnet/coreclr#9790)
* wip
* Adding Span API perf tests for comparison with arrays and to monitor progress
* Reducing iteration count so tests finish in < 2 minutes
* Reducing the number of test cases
* Commenting out ref DangerousGetPinnableReference test
* Updated csproj to use latest compiler
* Tuning test cases and runtime
Commit migrated from https://github.com/dotnet/coreclr/commit/
1bf3bbb44397875e94ae95b8527fd1928b5373fe