Many updates.

Fixed a bug when scrolling right with narrow columns.
Add clearing rows and columns.
Add inserting and deleting rows, columns, and cells.
Make the screen more responsive while navigating without scrolling.
Improve performance of loading files.
Increase the number of columns to 26.

Minor bug fix

Fix the bug where cell references weren't number-like and so a subtraction after a cell reference was treated as a unary negation.

It's now a not-terrible thing.

I added file save/load and made the edit line usable. I'm pretty sure I made it slower, because I had to make the cell store entries not a power of two.

A good starting point.

It does some things that can be described as useful. Without save functionality, the utility is questionable. Also, the Windows ConIO version is what is uploaded. It is objectively inferior to the curses version, but I don't want to muck about with statically linking curses.