Modified the Decompressor class so it no longer keeps
a ReadStream for its input data. This helps us avoid doing
a vtable lookup for every byte read, and halves the time
needed to decompress.
Rewrote most of the engine using a much more object-oriented approach and using more of ScummVM's common classes. This design deviates quite a lot from the original engine's, but should be more maintainable and extensible in the future.
