Support recursive public function calls

[spalladino]

At the moment, the sequencer only executes the topmost call stack item for a public function. We should keep running until we exhaust the call stack, similar to how the private kernel circuit works.

[iAmMichaelConnor]

Make sure to chat with @sirasistant who worked on private nested function calls a few weeks ago. :)

Edge case:

• If, during execution of a tx, some function (fn A) is called more than once, then the same public state could be read / written-to more than once. The public kernel circuit will need to handle this, by identifying and removing duplicate state reads/transitions from its call stack. I think we chatted about this last week. It was a fiddly problem for private calls, but I think you've already solved it for public calls.

[spalladino]

@iAmMichaelConnor spoke with him earlier today. This seems to be very similar to how private nested functions call indeed, we'll just need to add an extra oracle call and process the call stack items in the sequencer.

However, calling from private to public may be a bit more interesting, since it requires changing the Tx object. I created a new issue to discuss that: #447.