[bitboard] Fix the warning about shared references to mutable static data

I've lived with this warning for a long time because I didn't really understand
it.

```
warning: `chessfriend_core` (lib) generated 1 warning (run `cargo fix --lib -p chessfriend_core` to apply 1 suggestion)
warning: creating a shared reference to mutable static is discouraged
  --> bitboard/src/library.rs:66:9
```

I was able to fix this by creating a new type with a single OnceLock attribute.
The OnceLock acts as a cell, making it mutable, even if self is not. So, you can
declare the MoveLibraryWrapper non-mutable static, but still initialize the
library inside the Cell.

bitboard/src/library.rs

@@ -60,16 +60,8 @@ pub(crate) const LIGHT_SQUARES: BitBoard =
 pub(crate) const DARK_SQUARES: BitBoard = BitBoard(!LIGHT_SQUARES.0);
 
 pub(super) fn library() -> &'static MoveLibrary {
-    static mut MOVE_LIBRARY: OnceLock<MoveLibrary> = OnceLock::new();
-
-    unsafe {
-        MOVE_LIBRARY.get_or_init(|| {
-            let mut library = MoveLibrary::new();
-            library.init();
-
-            library
-        })
-    }
+    static MOVE_LIBRARY: MoveLibraryWrapper = MoveLibraryWrapper::new();
+    MOVE_LIBRARY.library()
 }
 
 macro_rules! library_getter {
@@ -80,6 +72,26 @@ macro_rules! library_getter {
     };
 }
 
+struct MoveLibraryWrapper {
+    library: OnceLock<MoveLibrary>,
+}
+
+impl MoveLibraryWrapper {
+    const fn new() -> Self {
+        Self {
+            library: OnceLock::<MoveLibrary>::new(),
+        }
+    }
+
+    fn library(&self) -> &MoveLibrary {
+        self.library.get_or_init(|| {
+            let mut library = MoveLibrary::new();
+            library.init();
+            library
+        })
+    }
+}
+
 #[derive(Debug)]
 pub(super) struct MoveLibrary {
     // Rays