Modern compression algorithms are better than zlib both in terms of
space and time. The jar format, used for e.g. omni.ja, addons, etc.
could benefit from using such modern algorithms, but the format only
allows a limited set of compression algorithms.
However, the format in itself is flexible, in that it can be extended
with arbitrary compression algorithms. This breaks compatibility with
programs like unzip, obviously, but we've never promised the files
shipped with Firefox will always remain "valid" zips (which they already
aren't, but they currently work with most zip readers).
With this change, we allow those archives to contain brotli streams,
using an arbitrary large value for the compression type in the Zip local
file header. This only allows to read such archives, but not to produce
them, and, for now, support for brotli streams is kept Nightly-only,
until everything is pieced together and we're happy to ship it.
