script: prototype mathematical expressions as control values
scripting: fix operator precedence

Let '*' and '/' precede '+' and '-'.
scripting: expressions: avoid hardcoded result type
scripting: operation processor: implement clean-up routine
scripting: destroy operation processor together with controlled MarSystem

This calls for a way to attach MarSystems to other MarSystems
so that the attached ones are deleted with those that they attach to.

In future, we could think of tighter integration of control expression
processing with the MarControl system itself, so as to avoid using
a specialized MarSystem for that purpose.