zkasm: add infrastructure for address → slot → address translations

[nagisa]

In zkASM memory is addressed in word/register-sized slots rather than bytes. There are quite a few places where we will receive bytes in the backend and will want to convert to slots, or places where we need to supply bytes but have slot count at hand. We should add methods/functions to implement these conversions so that they don't need to be open coded every time a need to convert arises.

[aborg-dev]

See #186 (comment) for our plan on enabling slot addressing for stack.