getState & restoreState do not consistently deep copy.

[milescrawford]

getState's documentation says that it makes a deep copy, but restoreState does not make a copy when you pass this back in.

So, if you store a state, restore a board to that state, and modify the board, your stored state is mutated. This makes it very difficult to maintain a history, or keep a permanently-stored state that you revert to more than once.

See this JS fiddle for an example: https://jsfiddle.net/czjxue6y/4/