All decompressors now have exactly the same signature (as a result,
each decompressor now has to check itself that the header type is
correct). This allows the decompressors to be stored in a simple
dictionary, which makes the lookup process much simpler.
In preparation for #3, the compressed resource data headers are parsed
and stored as proper objects. For now these objects are only used
internally by the decompression code, but in the future they can be
exposed.
