e-mail threading (conversations)
This module implements Jamie Zawinski's threading algorithm. Quoted comments (/* " .. " */) are
taken almost verbatim from the algorithm.
The entry point of this module is the Threads struct and its new method. It contains
ThreadNodes which are the nodes in the thread trees that might have messages associated with
them. The root nodes (first messages in each thread) are stored in root_set and tree
vectors. Threads has inner mutability since we need to sort without the user having mutable
ownership.