Mark endian_scalar as unsafe. (#6588)

* Mark endian_scalar as unsafe.

Also
- removed the deprecated flexbuffer slice from example
- fixed some cargo warnings

* Assertions and read_scalar made unsafe

* Clippy lints

* Add to Safety

Co-authored-by: Casper Neo <cneo@google.com>

rust/flatbuffers/src/vtable_writer.rs

@@ -40,14 +40,18 @@ impl<'a> VTableWriter<'a> {
     /// to the provided value.
     #[inline(always)]
     pub fn write_vtable_byte_length(&mut self, n: VOffsetT) {
-        emplace_scalar::<VOffsetT>(&mut self.buf[..SIZE_VOFFSET], n);
+        unsafe {
+            emplace_scalar::<VOffsetT>(&mut self.buf[..SIZE_VOFFSET], n);
+        }
         debug_assert_eq!(n as usize, self.buf.len());
     }
 
     /// Writes an object length (in bytes) into the vtable.
     #[inline(always)]
     pub fn write_object_inline_size(&mut self, n: VOffsetT) {
-        emplace_scalar::<VOffsetT>(&mut self.buf[SIZE_VOFFSET..2 * SIZE_VOFFSET], n);
+        unsafe {
+            emplace_scalar::<VOffsetT>(&mut self.buf[SIZE_VOFFSET..2 * SIZE_VOFFSET], n);
+        }
     }
 
     /// Gets an object field offset from the vtable. Only used for debugging.
@@ -57,7 +61,7 @@ impl<'a> VTableWriter<'a> {
     #[inline(always)]
     pub fn get_field_offset(&self, vtable_offset: VOffsetT) -> VOffsetT {
         let idx = vtable_offset as usize;
-        read_scalar_at::<VOffsetT>(&self.buf, idx)
+        unsafe { read_scalar_at::<VOffsetT>(&self.buf, idx) }
     }
 
     /// Writes an object field offset into the vtable.
@@ -67,7 +71,9 @@ impl<'a> VTableWriter<'a> {
     #[inline(always)]
     pub fn write_field_offset(&mut self, vtable_offset: VOffsetT, object_data_offset: VOffsetT) {
         let idx = vtable_offset as usize;
-        emplace_scalar::<VOffsetT>(&mut self.buf[idx..idx + SIZE_VOFFSET], object_data_offset);
+        unsafe {
+            emplace_scalar::<VOffsetT>(&mut self.buf[idx..idx + SIZE_VOFFSET], object_data_offset);
+        }
     }
 
     /// Clears all data in this VTableWriter. Used to cleanly undo a