New interface for Player and NormalFormGame creation

@kusanominal Good job, and good point of symmetric games.

Here's my proposal:

* `NormalFormGame` only allows *two-player* symmetric games.
  (Symmetric game with more than two players must be created with an array with ndim=N+1 or a list of Players.)
* Do not allow `Player` to be created with action sizes.
  (1-dim array input would be ambiguous.)

I also refactored `payoff_vector` and deleted `br_correspondence`.

If this PR is fine with you, press the "Merge pull request" button to merge it to the `N_players` branch.