Download Rust snapshots in make rather than configure.

When parallel make (`-j`) is used, this enables building non-Rust dependencies
while the Rust compiler is being downloaded.

On my laptop, from an empty build directory,
`~/projects/servo/configure --disable-optimize && make -j6`
goes from ~7 minutes to ~6 minutes.
The difference matches the download time.
Merge pull request #3096 from SimonSapin/make-snapshot

Download Rust snapshots in make rather than configure. r=@Manishearth
Implement WorkerGlobalScope.importScripts.