Implement pin

If a piece is blocking a check such as:
q -------N-----K (black queen in the same line as white Knight and White King)
The piece cannot be moved.

In this situation any movement of the piece (that doesn't eliminate the aggressor piece) is illegal and must not be taken into account in the list of possible moves.

This issue most probably requires #10 completed.

Some inspiration how to implement: [link](https://youtu.be/U4ogK0MIzqk?t=376)

Start by reading issue #10, which this work may depend on, then review the Rust chess-move generation entry points. Implement pin handling so moves that leave the king in check are excluded unless they remove the attacking piece, and verify that the possible-move list reflects this rule.

Written by the indexing model from the issue text.