SCORU/Node: defunctorize code with respect to PVM

Context

This MR removes the functors which take the PVM as argument. This simplifies the code a lot and removes some caveats (where one would have to take care to only instantiate once). This MR is also required to implement the protocol agnostic rollup node.

This MR essentially makes the following change: the instantiated PVM is kept in the "node context" (as a fist class module, see this line), which makes it then accessible wherever we need it without resorting to functors.

Manually testing the MR

dune exec tezt/tests/main.exe -- --file canary.ml