Decode(): Takes an encoded .huff file, decodes and writes decoded text to * an output file.

Encode(): Encodes a text file to a .huff file.

GetCompressionRatio(): Compares the sizes of the original and the compressed * files, returns the ratio as a float64.

PriorityQueue implemented using Heap interface in order to always be able to access the two trees withthe lowest total frequencies of their leaves.