Lazily Evaluated Formulas and useSheet composable

We pick up from the previous screencast, where we built a basic spreadsheet engine and UI. We will now add a useSheet composable to enable us to easily update the spreadsheet. We also implement basic support for the SUM formula, and discuss how we might support arbitrary complex formulas, as well as some of the limitations of our current design.

A text version of this screencast is available here. It was published on 20 Sep, 2020.