Our current way of tracking definition depth is confusing

[bugarela]

See https://github.com/informalsystems/quint/pull/1276#issuecomment-1832512374

We have currently 4 visitors that require information about depth of definitions. In the PR linked above, I decided to move most of the depth tracking to the walk* functions, in order to fix a problem that would otherwise require changes to all 4 visitor's implementation. However, the visitors still have the responsibility of initializing the depth counter, so this is not properly encapsulated.

The comment I linked lists some possible solutions.