Add storage_variables printer

[nisedo]

Add a printer for storage variables, which serves the same purpose as the entry-points printer: slither . --print storage-variables

[montyly]

For visibility, we have VariableOrder which is a bit similar. Should we keep both, or merge them?

[nisedo]

Current screenshot of the storage-variables printer:

Current screenshot of the variable-order printer:

[smonicas]

I'm not sure about merging this one as it would be almost an exact duplicate of the variable-order printer. I would rather improve variable-order.