SCORU: Wasm PVM: Handle the right number of inputs per commitment

Goal: The Wasm PVM handles the right number of inputs per commitment.

A commitment period is split into L parts, each corresponding to an inbox level with a non-zero amount of messages.

For each part of the commitment period, kernel_next may be called up to N (tbd) times. After which, the PVM defaults to consecutively reading inputs from L1 until reading an input from a subsequent inbox level. The loop continues after that.

This means each section of the commitment period, has a phase to process the previously accumulated and a phase to gather inputs for the next loop.