`repeat` terminal

[eelstork]

Ordered composites have the end and loop terminals.

• end finalizes the state of a composite, in that an ended sequence will always return done, whereas an ended selector will always return fail, until the underlying ordered composite is reset.
• Upon executing the last subtask, looping composites return cont, also resetting the underlying task iterator.

Currently we lack a repeat mode allowing a composite to reset the task iterator on success/failure. This is inconvenient because many tasks are repeatable.