clean up script engine methods

[essweine]

This PR

• moves the implementation of the call_service method to the scripting environment (though the engine itself will retain the method with a deprecation warning through the next release)
• remove the _execute and _evaluate methods in favor of just calling the environment directly.

This moves towards creating a clearer distinction between the how the script engine interacts with the workflow (which most people are likely to not need to extend) vs how exec and eval work (which people are likely to do), providing a unified extension mechanism (for most use cases).