RFC: adb: add zstd compression support

This adds zstd compression support for adb-using packages. The mkpkg applet is extended with new options compression, complevel and threads. On my machine, using the default settings (which is to use all CPU cores by default and compression level 3), I get 30 times faster package creation, as well as 20% smaller packages. As a side effect, this also enables changing the compression level for deflate-using packages when creating them with mkpkg.

Now I'd like to get this reviewed, to make sure the stream code is correct, as well as any other necessary changes.

Also, I'm not entirely sure about how to handle the compression parameters for adbsign. The input for adbsign might be a heterogeneous set of packages so they cannot use the same parameters.