In preparation for much more thorough optimization of task-graphs, this
makes a few changes:
* optimization is split into thre phases, with task removal in one phase
(following dependency links) and task replacement in the next (in the
reverse order).
* optimization uses class instances instead of functions for optimizations;
this allows different functions for different phases, and also leaves open
the possibility of composing optimizations.
* the replacement phase can also support removal; this is when utility tasks
like symbol uploads can be optimized away iff their parent task is
optimized.
MozReview-Commit-ID: C5QznNpwqXn
These both have a similar form, recursing over a nested JSON structure, and are
useful outside of the modules in which they are defined.
MozReview-Commit-ID: 1bsRtlaQol7
