Mutability, promises, and monads are misplaced

[clarksmr]

For historical reasons, the chapters/sections on mutability, promises, and monads are shoe-horned into the same chapter as efficient data structures. We are considering factoring out all of that and moving it to immediately after modular programming. That would let us cover the material in the "OCaml Programming" part of the textbook instead of the "Correctness and Efficiency" part. Locally at Cornell, it would improve the course schedule and possibly the final project.