Investigate better code sharing between valueDecoder and refWalker

[cmasone-attic]

There's some duplication right now, because these are performance-sensitive code pathways and avoiding the allocations that valueDecoder needs is a meaningful performance gain when one is just walking refs.

Still, it smells like there's a better approach here. We need to try some ideas and see if any will work while causing only small performance degradations.

FYI @arv @rafael-atticlabs