Improve the is_fen_valid function

[johndoknjas]

Note: this issue was originally posted in the old stockfish repo here: zhelyabuzhsky#125

Some discussion of this function in PR zhelyabuzhsky#124. If possible, would be good to make this function more accurate, although this may be challenging. The function works by checking if the fen syntax is valid, and also by giving a temporary SF executable the FEN and seeing if it crashes. This seems to work well for most use cases but it's not completely accurate.

See the discussion in zhelyabuzhsky#124 for a test case that the function is inconsistent with (it's a position with both kings next to each other: 8/8/8/3k4/3K4/8/8/8 b - - 0 1). If some improvements can be made for the function, this test case should ideally pass for any version of Stockfish being used.

In addition, the 2k2q2/8/8/8/8/8/8/2Q2K2 w - - 0 1 test case now fails for a recent Stockfish development build (May 30), since the is_fen_valid function incorrectly returns True. This is because SF now gives a bestmove, even though the position is illegal. See #41.