Fix #922: Bindings generator generates .pyc files
auto merge of #1735 : tgkokk/servo/python-write-bytecode, r=kmcallister

Forgot to actually claim #922. I hope no one is working on it.

I removed `PYTHONDONTWRITEBYTECODE=1` from every relevant file and I added code for removing `*.pyc` files in `clean-script` in `mk/clean.mk`. You might want to move the main logic into a function like `REMOVE_PYC_FILES`, since the files will exist eventually in other trees under `/src`. I used `find` in order to find all `*.pyc` files, since they might be in other subdirectories.

This is my first PR, so feel free to give me any feedback you might think is necessary.
Implement JSManaged for DOM objects.
auto merge of #1591 : jdm/servo/jsmanaged, r=Ms2ger
layout: Run assign-widths in parallel.

5% layout speedup on mobile Wikipedia.
layout: Combine assign-widths and assign-heights traversals.

15% layout speedup on mobile Wikipedia.
layout: Make `ParallelPreorderFlowTraversal` more generic
layout: Construct flows in parallel, without a leaf set
layout: Create a font context in sequential mode