[board] Implement a position! macro for creating Positions

For example:

```
let position = position![
    White King on E4,
    Black Rook on E8,
];
```

board/src/lib.rs

@@ -4,6 +4,7 @@ mod bitboard;
 mod moves;
 #[macro_use]
 pub mod piece;
+#[macro_use]
 pub mod position;
 mod square;
 

board/src/position/position.rs

@@ -18,6 +18,15 @@ pub(crate) enum BoardSide {
     Queen,
 }
 
+#[macro_export]
+macro_rules! position {
+    [$($color:ident $shape:ident on $square:ident,)*] => {
+        Position::from_iter([
+            $(piece!($color $shape on $square)),*
+        ].into_iter())
+    };
+}
+
 #[derive(Clone, Eq, Hash, PartialEq)]
 pub struct Position {
     color_to_move: Color,
@@ -199,6 +208,17 @@ impl Default for Position {
     }
 }
 
+impl FromIterator<PlacedPiece> for Position {
+    fn from_iter<T: IntoIterator<Item = PlacedPiece>>(iter: T) -> Self {
+        let mut position = Position::empty();
+        for placed_piece in iter {
+            _ = position.place_piece(placed_piece.piece(), placed_piece.square());
+        }
+
+        position
+    }
+}
+
 impl fmt::Debug for Position {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let mut output = String::new();