Auto merge of #45399 - alexcrichton:compress-parallel, r=michaelwoerister

Auto merge of #45399 - alexcrichton:compress-parallel, r=michaelwoerister

rustc: Move bytecode compression into codegen

This commit moves compression of the bytecode from the `link` module to the
`write` module, namely allowing it to be (a) cached by incremental compilation
and (b) produced in parallel. The parallelization may show up as some nice wins
during normal compilation and the caching in incremental mode should be
beneficial for incremental compiles! (no more need to recompress the entire
crate's bitcode on all builds)