Scatter-based scantables.

This gains about 0.2% on derf, 0.1% on hd and 0.4% on stdhd. I can put
this under an experimental flag if wanted, just trying to get my patch
queue in shape.

Change-Id: Ibe1a30fe0e0b07bec4802e0f3ff0ba22e505f576
3 files changed