fix: prevent circular references

[jimniels]

If somebody references the cell they're currently in, the app just gets stuck in an infinite loop with no way to exit except refresh the page

[davidkircos]

This is fixed in a individual cell, but you can still create a circular ref with two cells

[ddimaria]

I confirmed that individual cell reference are now caught. I can cause the infinite loop by:

1. Code cell at (0,0) that reads from cell (1,0)
2. Code cell at (1,0) that reads from cell (0,0)

I would expect at least the ability to cancel the execution, but would prefer to just generate an error in the code editor's console.

[luke-quadratic]

Can now cancel, marking to close pending discussion