backend: Add Round Class

[pangiann]

Round Class represents an active Round in the Hangman Game. Specifically, keeps track and updates the main state of the game comprised of:

• An active dictionary which inludes only words equal to the playing word with respect to: a. the length of the word b. the already shown letters
• A list of chars for each word's position with their probability to be the correct guess
• Number of total and correct guesses
• History of guesses
• The state of the guessing word.

In a nutshell, this class does the following in each round:

1. Removes unlike words in regard to the selected one
2. Computes the new probabilities for chars in each position
3. Computes the points of a winning or losing round

Refs: https://github.com/pangiann/hangman/issues/5