[bitboard] Write some documentation; mark some methods const

- Implement BitBoard::is_populated(), the opposite of ::is_empty()
- Write a bit of documentation for the BitBoard Library and for some methods on BitBoard
- Mark a few methods as const

bitboard/src/library.rs

@@ -1,5 +1,13 @@
 // Eryn Wells <eryn@erynwells.me>
 
+//! # The BitBoard Library
+//!
+//! This module implements a collection of commonly used BitBoards that can be
+//! looked up efficiently as needed.
+//!
+//! The `library()` method returns a static instance of a `Library`, which
+//! provides getters for all available BitBoards.
+
 use crate::BitBoard;
 use chessfriend_core::{Color, Direction, Square};
 use std::sync::OnceLock;