Fix pointer math.

The previous use of `get_unchecked` was UB according to
https://doc.rust-lang.org/nightly/std/primitive.slice.html#safety-2
and rust-lang/rust#120594 exposed this leading
to a panic caught by out tests as:
```
cargo test --all
...
thread 'fold::tests::ident_transformation_in_defs' panicked at library/core/src/panicking.rs:155:5:
unsafe precondition(s) violated: hint::assert_unchecked must never be called when the condition is false
thread caused non-unwinding panic. aborting.
error: test failed, to rerun pass `-p garando_syntax --lib`
...
```

Fixes rust-lang/rust#120910

Author: jsirois

garando_syntax/src/util/move_map.rs

@@ -29,13 +29,13 @@ impl<T> MoveMap<T> for Vec<T> {
             while read_i < old_len {
                 // move the read_i'th item out of the vector and map it
                 // to an iterator
-                let e = ptr::read(self.get_unchecked(read_i));
+                let e = ptr::read(self.as_mut_ptr().offset(read_i as isize));
                 let iter = f(e).into_iter();
                 read_i += 1;
 
                 for e in iter {
                     if write_i < read_i {
-                        ptr::write(self.get_unchecked_mut(write_i), e);
+                        ptr::write(self.as_mut_ptr().offset(write_i as isize), e);
                         write_i += 1;
                     } else {
                         // If this is reached we ran out of space