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

0 files changed, 0 insertions, 0 deletions