This compresses multiple files in parallel, and will split up larger
files (5MB+) into smaller chunks (1MB) to compress in parallel.
There is a small size overhead to recombine the chunks, but it's only a
few bytes per chunk, so for a 1MB chunk, it's minimal.
Rough numbers, with everything in the page cache, this can compress
~4GB (1000 files) down to 1GB in 6.5 seconds, instead of 120 seconds with
the non-parallel soong_jar and 150 seconds with zip.
Go's DEFLATE algorithm is still a bit worse than zip's -- about 3.5%
larger file sizes, but for most of our "dist" targets that is fine.
Change-Id: Ie4886c7d0f954ace46e599156e35fea7e74d6dd7
017d8937fa