[enhance]: Implement "Board" with a 1D vector rather than 2D

### Proposed feature

It would be nice to consolidate how the board is represented.

For faster indexing, and consistency, it would be nice to use a 1d vector and abstract indexing based on the board's width and height.

Since we know that the board will not be "Jagged", there is no need to have a 2d vector, and it is causing inconsistencies.

Look into some of these neat resources:
https://stackoverflow.com/questions/46546321/implementing-2d-vector-syntax-for-accessing-a-1d-vector
https://doc.rust-lang.org/std/ops/trait.Index.html

Start by tracing every place where the board is represented or indexed, then read Rust's std::ops::Index documentation and the linked 2D-vector indexing resource. Done means the board uses one non-jagged 1D vector, indexing is abstracted using its width and height, and the current representation inconsistencies are removed.

Written by the indexing model from the issue text.