Add multiple new functions related to Moves

[DXPower]

Trying to integrate this project into my own, I found what felt to be many missing features when trying to use the library. A lot of these were related to Moves and retrieving information easily, intuitively, and without unnecessary copies.

• Game.Move(), MoveStr() changed return type from err to (*Move, err). This is a breaking change to existing code that calls this function, however it is easily fixed by changing code from err := game.MoveStr("...") to _, err := game.MoveStr("..."). This is very useful for getting information about a move just made, such as the tags, promotion type, or piece moved (see next bullet)
• Move.MovedPiece() allows for external code to query what piece was moved in a move. This has the most changes in other files, but nothing major. Adding new fields to move_test.go allowed for testing this. Other changes had to be made to the Engine and Decode to make sure this information is included whenever Moves are instantiated. In the future, this code can also be used to simplify the Encode and tests in many places.
• Move.GetTags() allows for external code to query the tags all at once. This is useful for storing in a database or data analysis. The alternative to this is querying it one at a time, which takes several if statements.
• Game.MoveSquares() allows external code to initiate a movement between two Squares. This is useful to allow more flexible control over the board.
• Game.GetMove() allows for external code to query a specific move in the history of the game without copying the array. This is useful for getting the last move made, for example. The alternative to this is calling Moves() and then performing your own index calculation on it. Even worse, if you want to get the piece moved in the last move, you must call Positions() then get the same index as the move you got, then query the move.S1() square of that position. With this combined with the Move.MovedPiece() function, this action becomes trivial for a caller.
• NewSquareFromStr() allows for external code to convert strings into Squares. This makes it easier to make a Square if you only store the strings of the origin and destination cells. Without this function, you must write your own code to convert a string into a File and Rank index.

Tests have also been updated to test and incorporate these functions.

[notnil]

Thanks for the thoughtful additions and I definitely see how these would be useful, however I am avoiding breaking changes at this time. Also its much easier to accept simpler more orthogonal PRs than sweeping changes.