Fix #7: add execute command

This commit adds support for a command execute:

- param in [1, 2, 3];
- execute("solver " + str(param));

This commit takes the opportunity for some code refactoring:

• write(...) and execute(...) are no longer special rule forms, but are just functions that returns unit (aka void), and there is a single rule form - call(args); for calling functions that return unit.

• There is now an enum type SimpleRule for rule forms that have no child rules: assignments, in, include, calls.

• There is now a struct Context to store the context of the interpreter.