DNF 3: better performance and a move to C++

Ha! Yeah, it looks like I basically just cloned Ninja with bake. Hadn't heard of it before; thanks for the pointer. I'm still going to parallelize bake, probably tomorrow now that I turned my attention to it in response to your comment and found an easy way to do it.

The two systems are somewhat different, probably most notably in that a Bakefile calls its clients to generate the dependency tree, and will give a previous client's output as input to a later client for modification. Of course, I haven't actually used that for anything. Also the -sub feature looks unique, though I haven't actually written a baker.py that uses it yet, so perhaps it's harder to make use of than I suspected. No harm in keeping it around, though.

(Also yeah it's months later; sorry for not seeing this at the time.)