rustdoc: use smarter encoding for playground URL
The old way would compress okay with DEFLATE, but this version makes uncompressed docs smaller, which matters for memory usage and stuff like `cargo doc`.
Try it out: <https://play.rust-lang.org/?code=fn+main()+{%0Alet+mut+v+=+Vec::new();%0Av.push(1+/+1);%0Aprintln!(%22{}%22,+v[0]);%0A}>
In local testing, this change shrinks sample pages by anywhere between 4.0% and 0.031%
$ du -b after.dir/std/vec/struct.Vec.html before.dir/std/vec/struct.Vec.html
759235 after.dir/std/vec/struct.Vec.html
781842 before.dir/std/vec/struct.Vec.html
100*((759235-781842)/781842)=-2.8
$ du -b after.dir/std/num/struct.Wrapping.html before.dir/std/num/struct.Wrapping.html
3194173 after.dir/std/num/struct.Wrapping.html
3204351 before.dir/std/num/struct.Wrapping.html
100*((3194173-3204351)/3204351)=-0.031
$ du -b after.dir/std/keyword.match.html before.dir/std/keyword.match.html
8151 after.dir/std/keyword.match.html
8495 before.dir/std/keyword.match.html
100*((8151-8495)/8495)=-4.0
Gzipped tarball sizes seem shrunk, but not by much.
du -s before.tar.gz after.tar.gz
69600 before.tar.gz
69480 after.tar.gz
100*((69480-69600)/69600)=-0.17
d68b5a42e2