Use similar compression settings as before updating to use flate2
Fixes#42879
(My first PR to rust-lang yay)
This changes the compression settings back to how they were before the change to use the flate2 crate rather than the in-tree flate library. The specific changes are to use the `Fast` compression level (which should be equivialent to what was used before), and use a raw deflate stream rather than wrapping the stream in a zlib wrapper. The [zlib](https://tools.ietf.org/html/rfc1950) wrapper adds an extra 2 bytes of header data, and 4 bytes for a checksum at the end. The change to use a faster compression level did give some compile speedups in the past (see #37298). Having to calculate a checksum also added a small overhead, which didn't exist before the change to flate2.
r? @alexcrichton
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a1f180bde3