Instantiate chessboard with FEN string

[krestenlaust]

Why?
This would make test code much nicer when setting up specific position.
It also allows setting up the board on Lichess, then simply "importing" it.

Where?
Probably just directly in the Chessboard class.

Documentation: https://www.chess.com/terms/fen-chess

[krestenlaust]

Should probably 'only' support 8x8 boards, otherwise the FEN would be nonstandard

[krestenlaust]

This should also make the transposition table much more memory efficient, storing only a FEN-string as key, as opposed to a whole object.

[krestenlaust]

It should be possible to make an extended FEN-standard for non-standard boardsizes, this would allow for general improvement of bots.