Remove the empty constructor

[treeowl]

• Remove the empty constructor, allowing GHC to unbox vectors.

• Make strict folds a bit stricter and force them to inline, which lets GHC optimize filter, partition, and reverse to avoid allocating a fresh root node on every snoc. The folds now match list folds rather than the default Foldable definitions. The inlining isn't free, but it seems pretty important. Also use List.reverse just a tad in foldl' and foldr to avoid likely-unproductive muck in the Core.

• Add Foldable method definitions.

• Use "unsafe" shifts.

• Break up snoc so the common case can inline.

• Make index catch negative indices as well as ones that are too large.

• Define // in terms of update, rather than the other way around.

• Define reverse using foldr' rather than working via a list.

• Unpack the vectors in the intermediate result of partition.

• Test unsafe indexing much more aggressively by testing all indices into the generated vector instead of just one.

• Let the Arbitrary instances produce small vectors more easily; we can then set up CI to run the test suite with multiple size settings to catch any issues that might arise with small or large vectors.

[treeowl]

@travitch, I think this is ready for review.

[travitch]

Thanks, this is great! I appreciate you looking through the core to see what is really going on. I know that I had no idea how to do that when I first wrote this code.

I'm trying to remember why there was even an Empty constructor. It seems obviously unnecessary now.