add ptr_offset_by_literal clippy lint and set to deny

src/header/stdio/mod.rs

@@ -478,7 +478,7 @@ pub unsafe extern "C" fn fgets(
         let unget_read_size = cmp::min(left, stream.unget.len());
         for _ in 0..unget_read_size {
             unsafe { *out = stream.unget.pop().unwrap() as c_char };
-            out = unsafe { out.offset(1) };
+            out = unsafe { out.add(1) };
         }
         left -= unget_read_size;
     }

src/header/stdio/scanf.rs

@@ -471,7 +471,7 @@ pub unsafe fn inner_scanf<T: Kind>(
                     {
                         if let Some(ref mut ptr) = ptr {
                             unsafe { **ptr = character as c_char };
-                            *ptr = unsafe { ptr.offset(1) };
+                            *ptr = unsafe { ptr.add(1) };
                             data_stored = true;
                         }
                         // Decrease the width, and read a new character unless the width is 0