Side: Make inversion function be associated

programs/mango-v4/src/state/mango_account.rs

@@ -492,7 +492,7 @@ impl MangoAccount {
             .0;
         // pa.settle_funding(cache);
 
-        let side = invert_side(fill.taker_side);
+        let side = fill.taker_side.invert_side();
         let (base_change, quote_change) = fill.base_quote_change(side);
         pa.change_base_position(perp_market, base_change);
         let quote = I80F48::from_num(

programs/mango-v4/src/state/orderbook/order_type.rs

@@ -54,10 +54,11 @@ pub enum Side {
     Ask = 1,
 }
 
-pub fn invert_side(side: Side) -> Side {
-    if side == Side::Bid {
-        Side::Ask
-    } else {
-        Side::Bid
+impl Side {
+    pub fn invert_side(self: &Side) -> Side {
+        match self {
+            Side::Bid => Side::Ask,
+            Side::Ask => Side::Bid,
+        }
     }
 }