So far an external function could not return any value (more precisely, return values were actually ignored, and 3D type-checked an external function call as returning unit even if the user stated a non-void return type in the declaration of the external function.)

This PR fixes that issue, by:

• allowing non-unit return type in the extern_action combinator in the prelude
• taking the user-declared external function return type into account in the 3D type-checker

With this PR, an action can, for instance, call an external function and decide to succeed or fail depending on its return value. (This was long due; until now, one had to use out-parameters to painfully mimic this behavior.)

It also:

• adds an array iterator example that calls external functions to populate an output array
• documents external function calls in the 3D manual