Day 23 Part 6 - Complete P84 minimalSpanningTree

To ensure that this problem is even well defined, we subclassed Graph[T, U]
to form GraphInt[U]. This ensures that all the edge labels are integers, which
then allows us to talk about minimal spanning trees.

A new object, GraphStringInt, is created. It has a fromStringLabel method to
convert a string representation of a graph to GraphInt[String], which may
later be used to compute a minimal spanning tree.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>