`tree_map` takes a tree and a function that transforms old leaves into
new leaves, and maps that function over all the leaves in the tree,
leaving the structure of the tree unchanged. For instance:
let double i = i + i;;
`tree_map` takes a tree and a function that transforms old leaves into
new leaves, and maps that function over all the leaves in the tree,
leaving the structure of the tree unchanged. For instance:
let double i = i + i;;