Plan transformations: task trees, code replacement, and serialization

Description: the purpose of this tutorial is to present an in-development aspect of CRAM; expect content to change. It contains an intro to task trees, a data structure used by CRAM functions (note: this has existed in CRAM for a while and will not change). It also describes code replacement (also long existing), failure handling through code replacement (new), and serialization of task trees and functions (new).