Improve memory usage of Actions

[J08nY]

When simulation is used on a large scale ocassionally one wants to store all actions in a context for later analysis (i.e. all IVs of potentially many executions). This results in signficant memory usage that should be improved.

Some of the memory usage is unavoidable. For example a formula application may have about 15 operations, each of which needs to store a field element. Plus other metadata that is stored in the actions necessarily takes up some space.

Some ideas to reduce it:

• Use __slots__.
• Simplify the FormulaAction attributes (some info may be duplicated due to the API, maybe turn some into properties?).