Add stress tests for Span<T> APIs

[ahsonkhan]

• Tests with multiple threads to confirm that Span APIs are stateless and thread safe
• Tests with lots of GC allocations to confirm that when GC moves addresses around, the use of Span is still correct

[KrzysztofCwalina]

@ahsonkhan, is that something we should track in corefx now?

[ahsonkhan]

Tracking this issue in https://github.com/dotnet/corefx/issues/28227 now. Closing.