The old names ("system" and "application" compression) were not really
accurate in all cases, so the compression types are now referred to by
their number.
The non-stream-based APIs still exist as before and are not deprecated,
they just act as thin wrappers around the stream-based API.
The main rsrcfork module doesn't use the stream-based APIs yet, because
it reads each resource's data all at once and not incrementally.
This is a little more complex than with the other decompressors,
because .dcmp2 has to behave differently when at the byte before EOF.
Checking whether this is the case requires lookahead, which is not easy
to do with a plain IO stream.
Some buffered IO streams provide a peek method for lookahead, but
others don't (such as io.BytesIO). There is no standard way to wrap an
already buffered IO stream to add a peek method, so we need a custom
wrapper class and helper function for this purpose.
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.
