refactor: recursively replace ExprRef with SignalRef

[domoritz]

Together with #7438, this will allow us to have stricter types and also refactor a bunch of code

[domoritz]

After this, I think we can remove a lot of the signalRef methods in common.ts.

As a follow up, look at these PRs to see what we can revert: #6825, #6826, #6847, #6849, #6850, #6806, and #6805.