feat: implement Clone for PageTable (#236)

Author: toku-sa-n

src/structures/paging/page_table.rs

@@ -177,8 +177,12 @@ const ENTRY_COUNT: usize = 512;
 ///
 /// This struct implements the `Index` and `IndexMut` traits, so the entries can be accessed
 /// through index operations. For example, `page_table[15]` returns the 15th page table entry.
+///
+/// Note that while this type implements [`Clone`], the users must be careful not to introduce
+/// mutable aliasing by using the cloned page tables.
 #[repr(align(4096))]
 #[repr(C)]
+#[derive(Clone)]
 pub struct PageTable {
     entries: [PageTableEntry; ENTRY_COUNT],
 }