Speed up fromList

[treeowl]

fromList creates a bunch of churn at the root of the structure. If we give up on list fusion (which only helps when the argument is a good producer, and doesn't help enough), we can easily built a forest from chunks of, say, eight elements each. Then just finish up the job. When the argument is a good producer, we could try to fuse with it a bit more cleverly; I'm not sure how far the optimizer will let us take that.

[treeowl]

Also: for fromList, we can use an extra-eager version of insertion that doesn't build any thunks. That probably doesn't help a lot, but should help a little.

[treeowl]

In #26, I reimplement fromList using a function for converting a list to a forest, and being extra-eager. This feels like the Right Thing, since it saves a comparison for every element and is basically free. That approach will certainly work with chunking too.