tic tac toe - contract

Purpose

The goal is to write clean code that can be an initial guideline to develop using eosio.cdt.

This is a modified version of the following contract.

• https://github.com/EOSIO/eos/tree/master/contracts/tic_tac_toe

I converted the existing tic_tac_toe contract from eosio with reference to the following document.

• https://eosio.github.io/eosio.cdt/1.6.0/upgrading/1.2-to-1.3.html
• https://eosio.github.io/eosio.cdt/1.6.0/upgrading/1.5-to-1.6.html