index.write_tree: now uses MemoryDB, making tree handling more efficient as IO will only be done when required. A possible disadvantage though is that time is spent on compressing the trees, although only the raw data and their shas would theoretically be needed. On the other hand, compressing their data uses less memory. An optimal implementation would just sha the data, check for existance, and compress it to write it to the database right away. This would mean more specialized code though, introducing redundancy. If IStreams would know whether they contain compressed or uncompressed data, and if there was a method to get a sha from data, this would work nicely in the existing framework though