[explorer] Add a load command

Loads a board position from a FEN string.

Plumb through setting the Zobrist state on an existing board. Rebuild the hash
when setting the state.

position/src/position.rs

@@ -2,6 +2,7 @@
 
 mod captures;
 
+use crate::fen::{FromFenStr, FromFenStrError};
 use captures::CapturesList;
 use chessfriend_bitboard::BitBoard;
 use chessfriend_board::{
@@ -230,6 +231,10 @@ impl Position {
     pub fn zobrist_hash(&self) -> Option<u64> {
         self.board.zobrist_hash()
     }
+
+    pub fn set_zobrist_state(&mut self, state: Arc<ZobristState>) {
+        self.board.set_zobrist_state(state);
+    }
 }
 
 impl Position {
@@ -238,6 +243,15 @@ impl Position {
     }
 }
 
+impl FromFenStr for Position {
+    type Error = FromFenStrError;
+
+    fn from_fen_str(string: &str) -> Result<Self, Self::Error> {
+        let board = Board::from_fen_str(string)?;
+        Ok(Position::new(board))
+    }
+}
+
 impl ToFenStr for Position {
     type Error = <Board as ToFenStr>::Error;