Use TaskBuilder to spawn worker threads.
Use TaskBuilder for the main Servo task.
remove unnecessary clone usage
Merge pull request #3001 from jxs/master

remove unnecessary clone usage
Merge pull request #2998 from Ms2ger/taskbuilder

Use TaskBuilder where possible; r=metajack
Merge pull request #3002 from servo/rustdoc-layout

Un-blacklist the layout crate from rustdoc.
Generate rustdoc documenation for submodules.