Add new problem module prob757 and update mod.rs to include it

- Introduced the `prob757` module, which is now part of the Project Euler series.
- Updated the `mod.rs` file to include the new `prob757` module, ensuring it is accessible within the project.

src/project_euler/mod.rs

@@ -11,6 +11,7 @@ pub mod prob686;
 pub mod prob700;
 pub mod prob719;
 pub mod prob751;
+pub mod prob757;
 pub mod prob800;
 pub mod prob808;
 pub mod prob816;