v0.4.1: engage mixins before batch processing

mixins may be (and are in one use case) relied upon during batch
operations. This fix engages mixins before returning the batch tree.
v0.5.0: implement `node.removeAll`

`removeAll` removes an array of child nodes from a node, immediate
children only. The children may be from any clone.
v0.5.1: faster implementation for removeAll

See the new benchmark for speed detail. On my machine, unbatched
removeAll w/ 1024 nodes happens in 0.2 seconds. Batched removeAll w/
1024 nodes happens in 0.125 seconds. Batch is "less safe", and
unbatched is now probably acceptable for most uses (mine, anyway).