Implement tree printing to avoid maximum recursion depth error

[harryrichman]

Printing a Tree object relies on the _asciiArt method that is implemented using pure recursion, which can result in a RecursionError for a sufficiently large tree. If the implementation is modified to use memoization, then this recursion error could be avoided.

Example:

t = Tree("(0);")
for i in range(1001):
    (t&str(i)).add_child(name=str(i + 1))
print(t)