Do not log CSS parse errors to stderr by default.

Fix #2083. Change the log level of CSS errors from error to info.
Remove a workaround for a Rust bug.

We’ve upgraded to a Rust version that fixed mozilla/rust#13322.
Optimize CSS error logging: check log level before task-local silencing.
Make ComputedStyle fields private and add getters.

This isolates layout code from upcoming refactoring in properties.rs.mako.
Un-disable and fix some warnings in the style crate.
Remove CowArc and use the new Arc::make_unique() method instead.
Remove *() hack now that rust upgrade is complete. Fixes memory corruption bug #2412.
Remove the workaround for Rust issue 10683.
auto merge of #2483 : zwarich/servo/remove-10683-workaround, r=jdm
Eliminate pointless copies of ImageResponseMsg in the image task.
auto merge of #2485 : zwarich/servo/eliminate-copy, r=jdm
auto merge of #2476 : glennw/servo/issue-2412, r=huonw

...on bug #2412.
Use *mut T for the T* pointers in SpiderMonkey.
Pass an immutable pointer to the private in NewProxyObject.
Make GetProtoOrIfaceArray return the actual type.
auto merge of #2487 : Ms2ger/servo/sm-mut, r=jdm
Remove *() hack now that rust upgrade is complete. Fixes memory corruption bug #2412.
make servo::run() and opts::Opts.n_render_threads public for use in embedding crate
fix build of embedding crate

I blame @larsbergstrom for this failure.
auto merge of #2445 : SimonSapin/servo/css-warnings, r=jdm

This is on top of the #2433 Rust upgrade.