Make more readability refactorings

src/board/fen.rs

@@ -12,7 +12,7 @@ pub fn from_fen(fen: &str) -> Result<Game, FenError> {
 
     let castling_ability = castling_ability(fen_parts[2])?;
 
-    let en_passant_target_square = en_passant_target_square(fen_parts[3])?;
+    let en_passant_square = en_passant_square(fen_parts[3])?;
 
     let halfmove_clock = halfmove_clock(fen_parts[4])?;
 
@@ -21,7 +21,7 @@ pub fn from_fen(fen: &str) -> Result<Game, FenError> {
     board.set_state(State::load_state(
         side_to_move,
         castling_ability,
-        en_passant_target_square,
+        en_passant_square,
         halfmove_clock,
         fullmove_counter,
     ));
@@ -123,7 +123,7 @@ fn castling_ability(castling: &str) -> Result<[Castle; 2], FenError> {
 
 use std::collections::HashMap;
 
-fn en_passant_target_square(square: &str) -> Result<Option<usize>, FenError> {
+fn en_passant_square(square: &str) -> Result<Option<usize>, FenError> {
     let mut sqr = square.chars();
 
     let mut files: HashMap<char, usize> = HashMap::new();