Add size hint for Chain

src/root.zig

@@ -320,6 +320,10 @@ pub const adapters = struct {
             pub fn next(self: *Self) ?Item {
                 return if (self.first.next()) |x| x else self.second.next();
             }
+
+            pub fn sizeHint(self: *const Self) usize {
+                return self.first.sizeHint() + self.second.sizeHint();
+            }
         };
     }