+ * - Idea: Simplify the stryucture, so that the first block is always diffs.
+ * On small blocks, that is all we have. Once a block has been merged, we
+ * allocate the first main block and a (new) firstblock ffor diffs. From
+ * that point on the word has two blocks for it. Order the allocations
+ * right, and you should always get sequential reads!
+ * - On allocating more blocks (in append), check the order of blocks, and
+ * if needed, swap them.