Rename align() to gap() to avoid confusion

Rename align() to gap() to avoid confusion

The name "align" made readers believe it returns the next multiple,
while in fact it's returning how much we need to get there.

The name "gap" was suggested to me, and I haven't found better. A fully
descriptive name would likely be quite long, and wouldn't preclude the
need to look the definition up anyway. (And I suspect even now one could
guess from context.)