Use concrete value for quad.Quad use

This is a thinking PR. The benefits are that code is simpler and there are performance improvements in most cases. However, we see a really significant regression in the pathological cases.

After [discussion on golang-nuts](https://groups.google.com/d/topic/golang-nuts/Cgc2sEhAjlU/discussion) I think it's likely due to tree traversal in GoLLRB and the stack size changes that that forces. This may be improved when I get around to using cznic/b, so I won't push on this PR, but I would like to get @barakmich's view on this kind of decision. It probably also suggests that we need to make leveldb-dependent benchmarks too.