[position] Rever the const-ness of Position's initializers

Apparently you can't actually do that. :( You can't call trait methods in const
contexts.

position/src/position/position.rs

@@ -24,12 +24,12 @@ pub struct Position {
 }
 
 impl Position {
-    pub const fn empty() -> Position {
+    pub fn empty() -> Position {
         Default::default()
     }
 
     /// Return a starting position.
-    pub const fn starting() -> Self {
+    pub fn starting() -> Self {
         const BLACK_PIECES: [BitBoard; 6] = [
             BitBoard::new(0b0000000011111111 << 48),
             BitBoard::new(0b0100001000000000 << 48),