Consolidation consolidation

[frankmcsherry]

Some improvements around consolidation.

First, the consolidate_container method was made into a trait method on ConsolidateLayout, which it required anyhow. This provides a default implementation, but allows other implementors to provide their own optimized implementation. For example, Vec<(D, T, R)> can simply consolidate in place and then mem::swap, which it does. This should make the chunkers based on ConsolidateLayout appropriate for Vec. I also renamed it consolidate_into, which made more sense once it took a &mut self argument.

Second, the implementation of consolidate_into was streamlined. It maintained an Option of the previous key and diff, but always sets this in the first iteration. We can simply take the first step and then maintain a non-optional previous key and diff. Minor, but should be simplifying.

[antiguru]

Nice!