Investigate changing string manipulation to use `Span<char>`

[mrlacey]

This shuold provide a performance improvement.

There isn't ATM an issue with the performance but as a lot of what the extension does is repeatedly happening on the UI Thread, so any performance improvement would help us be a good citizen of VS and avoid wasting resources or contributing to an unnecessary slowdown.

Try to make any changes in a way that preserves the readability of the current code.

Use https://benchmarkdotnet.org/articles/overview.html to measure and report the difference.

[mrlacey]

Details of investigation at https://www.mrlacey.com/2018/11/some-quick-notes-on-converting-string.html