Make slow things less slow

[treeowl]

It's a bit annoying that drop and append allocate intermediate lists of individual elements. We can break up a vector (or part of one) into a type like this:

data Blobs a
  = ConsBlob !(Array a) (Blobs a)
  | EndBlobs ![a]

Now we can walk a Blobs list two chunks at a time to realign it. This is expensive, but it should be much less expensive than working through lists of elements.