Typed prophecy variables + cleanup

This MR introduces the following changes:

• new folders theories/proph and theories/proph/lib,
• things unrelated to logical atomicity moved from theories/logatom to the new folders,
• a more complete typed prophecy interface (supporting one-shot prophecies) that is now usable,
• examples using the typed prophecy interface (lazy coin + clairvoyant coin),
• specs for all the coin examples, eager implementation for the eager spec.

@jung: the coin examples of the Iris repository are included in a modified version, mainly linking them to their spec.