Block on required update

Should block an add if it requires an update on an in-file dependency.

scratch/main> builtins.merge

x = 1

scratch/main> add

Update x, and add a new y which depends on the update

x = 10
y = x + 1

Try to add only the new y. This should fail because it requires an update to x, but we only ran an 'add'.

scratch/main> add y