[board] Move position builders into a builders module

Move PositionBuilder to position_builder.rs and export it from the builders module.

board/src/position/builders.rs

@@ -1,83 +0,0 @@
-// Eryn Wells <eryn@erynwells.me>
-
-use super::{flags::Flags, piece_sets::PieceBitBoards};
-use crate::{
-    piece::{PlacedPiece, Shape},
-    square::Rank,
-    BitBoard, Color, MakeMoveError, Move, Piece, Position, Square,
-};
-use std::collections::BTreeMap;
-
-#[derive(Clone)]
-pub struct Builder {
-    player_to_move: Color,
-    flags: Flags,
-    pieces: BTreeMap<Square, Piece>,
-    kings: [Square; 2],
-}
-
-impl Builder {
-    pub fn new() -> Self {
-        Self::default()
-    }
-
-    pub fn place_piece(&mut self, piece: PlacedPiece) -> &mut Self {
-        let square = piece.square();
-
-        if piece.shape() == Shape::King {
-            let color_index: usize = piece.color() as usize;
-            self.pieces.remove(&self.kings[color_index]);
-            self.kings[color_index] = square;
-        }
-
-        self.pieces.insert(square, piece.piece());
-
-        self
-    }
-
-    pub fn build(&self) -> Position {
-        let pieces = PieceBitBoards::from_iter(
-            self.pieces
-                .iter()
-                .map(PlacedPiece::from)
-                .filter(Self::is_piece_placement_valid),
-        );
-
-        Position::new(self.player_to_move, self.flags, pieces, None)
-    }
-}
-
-impl Builder {
-    fn is_piece_placement_valid(piece: &PlacedPiece) -> bool {
-        if piece.shape() == Shape::Pawn {
-            // Pawns cannot be placed on the first (back) rank of their side,
-            // and cannot be placed on the final rank without a promotion.
-            let rank = piece.square().rank();
-            return rank != Rank::One && rank != Rank::Eight;
-        }
-
-        true
-    }
-}
-
-impl Default for Builder {
-    fn default() -> Self {
-        let pieces = BTreeMap::from_iter([
-            (
-                Square::KING_STARTING_SQUARES[Color::White as usize],
-                piece!(White King),
-            ),
-            (
-                Square::KING_STARTING_SQUARES[Color::Black as usize],
-                piece!(Black King),
-            ),
-        ]);
-
-        Self {
-            player_to_move: Color::White,
-            flags: Flags::default(),
-            pieces: pieces,
-            kings: Square::KING_STARTING_SQUARES,
-        }
-    }
-}