Add support for local font faces. Improves fonts on rust lang and guide.
script: Use atom comparison in more places, especially for attributes.

75% improvement in style recalc for Guardians of the Galaxy.
auto merge of #3675 : glennw/servo/local-fonts, r=pcwalton
auto merge of #3623 : pcwalton/servo/use-atoms-2, r=jdm

75% improvement in style recalc for Guardians of the Galaxy.
Add a to_js method to the casting trait code in CodegenRust.py
Replace the manual checks and calls to transmute_copy in /layout/wrapper.rs with calls to to_js

Fixes #3616