Move some very-high-level rationale-type stuff to new file rationale.md. New section for the pipeline. Note that since refactoring eval() to use @onFulfill, there is now exactly one call to each of the five loader hooks in the implementation, and they appear in the correct order, one by one, in class Loader.
Rewrite header comment for the first major section, "Loading and running code".
Move import() up top.
Move load().
Move eval to last.
Move comments around.
Fix a bug in import, at the cost of some complexity.