feat: standardize rounding in favour of swapper (#246)

* feat: standardize rounding in favour of swapper

This commit changes decay to standardize rounding in favour of swapper
- rounding output amounts up and input amounts down

* feat: remove fixedpoint

src/lib/DutchDecayLib.sol

@@ -43,7 +43,7 @@ library DutchDecayLib {
                 if (endAmount < startAmount) {
                     decayedAmount = startAmount - (startAmount - endAmount).mulDivDown(elapsed, duration);
                 } else {
-                    decayedAmount = startAmount + (endAmount - startAmount).mulDivDown(elapsed, duration);
+                    decayedAmount = startAmount + (endAmount - startAmount).mulDivUp(elapsed, duration);
                 }
             }
         }