Make a class BinTree that represents a binary tree.
The nodes of the tree contain int values,
and they don't have to refer to the parent.

1a. Make a constructor that creates a tree that consists of a single leaf.
1b. Make a constructor that puts a tree together from its two sides and a value for the top node.

Solve the following exercises using recursion:

2a. Make a toString operation for the tree.
    The output should look similar to this:   (((-, 5, -), 6, -), 6, -)
2b. Make another version of toString which formats the trees like this:

6
    -
    -
6
    -
    -
6
    6
        5
            -
            -
        -
    -

2c1. Make a function `sum` that returns the sum of the values in the tree.
2c2. Make a function `depth` that returns the length of the longest path from root to leaf in the tree.
2c3. Make a function `mirror` that returns a tree in which the left and right sides are reversed as compared to the original.
2c4. Make a function `replace` that takes two numbers, and returns a tree that looks just like the original, except that all instances of the first number are replaced by the second.