Implement en passant

If you have a white/black pawn in the 5/4 rank and the opponent moves a pawn to its side (by executing a two-square movement), you are allowed to eat that pawn, moving your to the 6/3 rank, diagonally.
It can only be done the turn after the opponent moved the pawn from its original squared.

For more [info](https://support.chess.com/article/683-what-is-en-passant).

Start by locating the pawn-movement and turn/state-handling code; the issue names no files or tests. Read the linked en passant rules, then trace how a pawn's previous move is represented. Done means a pawn can capture an opponent's just-moved two-square pawn diagonally on the immediately following turn, and cannot do so later.

Written by the indexing model from the issue text.