Implement part of a new Builder using the type state pattern

The API for this is much easier to use.

moves/src/builder.rs

@@ -0,0 +1,105 @@
+// Eryn Wells <eryn@erynwells.me>
+
+use crate::{castle, Move};
+use chessfriend_core::{PlacedPiece, Square};
+
+pub trait Style {}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub struct Builder<S: Style> {
+    style: S,
+}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub struct Null;
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub struct Push {
+    from: Option<Square>,
+    to: Option<Square>,
+}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub struct Capture {
+    push: Push,
+    capture: Option<Square>,
+}
+
+pub struct Castle {
+    castle: castle::Castle,
+}
+
+impl Style for Null {}
+impl Style for Push {}
+impl Style for Capture {}
+impl Style for Castle {}
+
+impl Builder<Null> {
+    pub fn new() -> Self {
+        Self { style: Null }
+    }
+
+    pub fn piece(piece: &PlacedPiece) -> Builder<Push> {
+        Builder {
+            style: Push {
+                from: Some(piece.square()),
+                to: None,
+            },
+        }
+    }
+
+    pub fn castling(castle: castle::Castle) -> Builder<Castle> {
+        Builder {
+            style: Castle { castle },
+        }
+    }
+
+    pub fn build(&self) -> Move {
+        Move(0)
+    }
+}
+
+impl Builder<Push> {
+    pub fn from(mut self, square: Square) -> Self {
+        self.style.from = Some(square);
+        self
+    }
+
+    pub fn to(mut self, square: Square) -> Self {
+        self.style.to = Some(square);
+        self
+    }
+
+    pub fn capturing(self, square: Square) -> Builder<Capture> {
+        Builder {
+            style: Capture {
+                push: self.style,
+                capture: Some(square),
+            },
+        }
+    }
+
+    pub fn capturing_piece(self, piece: PlacedPiece) -> Builder<Capture> {
+        Builder {
+            style: Capture {
+                push: self.style,
+                capture: Some(piece.square()),
+            },
+        }
+    }
+}
+
+impl Builder<Castle> {
+    pub fn build(&self) -> Move {
+        Move(self.style.into_bits())
+    }
+}
+
+impl Castle {
+    fn into_bits(&self) -> u16 {
+        match self.castle {
+            castle::Castle::KingSide => 0b10,
+            castle::Castle::QueenSide => 0b11,
+        }
+    }
+}