Function blocks_for 

pub fn blocks_for(clusters: usize) -> usize

The blocks a leaf chain of n clusters needs beyond the ones the program already had.

Here so that a test can say the number rather than count it, and so that whoever writes the jump table has one place to compare against. A switch that goes to a tree needs more than this, since the tree’s own nodes are blocks too, and a test that cares about one of those counts them.