WIP workshop

[jaekwon]

Imported manfred (moul)'s tictactoe and fixed the tests.

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These are internal notes not related to the workshop.

• TODO create another issue to see why defer m.injectLocOnPanic() doesn't work (see XXX).
• TODO related to above, make clearer error message when trying to pass a Gno struct with unexposed fields to fmt.Sprintf etc, so user knows to avoid t.Errorf("", x) but replace with t.Errorf("", x.String()) etc.
• TODO related to the above, maybe make t.Errorf() not use fmt.