Investigate and benchmark Span<T> usage for string operations

[daleholborow]

Adapt existing string-based logic to use Spans and implement performance comparisons.

Possible useful links:

https://github.com/dotnet/corefx/blob/master/src/Common/src/CoreLib/System/Text/ValueStringBuilder.cs

https://github.com/discostu105/stringbuilder-tests/blob/master/StringBuilderTests/Program.cs#L105