add Expression::original()

[gdesmott]

Checklist

• I have read the Contributor Guide
• I have read and agree to the Code of Conduct
• I have added a description of my changes and why I'd like them included in the section below

Description of Changes

add Expression::original()

My parser is storing an Expression and may have to retrieve the original string later when producing errors.

[Jake-Shadle]

Thanks! Are you planning on doing more changes? If so I'll hold off doing releases for each small PR.

[gdesmott]

I think I'm good now, but let me come back to you once I'm done implementing my thing. Should be ready soon. :)

[gdesmott]

All good, I managed to implement my exact use case!

Thanks a lot for the very quick reviews and releases.