Rewrote decompression to use chained decoders, creating a stream and
avoiding in-memory decompression, which caused redundant copying.
Now ouch supports any number of extensions as decompressing formats, not
only two.
Those formats won't be supporte by now, at least for compressing.
Archive formats are are only meant to be used as the first extension,
because archives are meant to be used with multiple files.
