AVM circuit: Handling variable length memory reads + writes

[IlyasRidhuan]

Many gadget operations, e.g hash permutation function, operates on a slice of information as inputs and/or output.

These slices are likely to be more than our number of current registers (4). Instead of adding more registers, we should be able to dynamically read and write these slices from memory into the gadgets (where the gadget specifies the logic)