Implement Debug for a couple structs.

I am finding it useful to be able to pretty print Mappers and
ActivePageTable structs.

Signed-off-by: Wren Turkal <wt@penguintechs.org>

src/arch/x86_64/paging/mapper.rs

@@ -76,6 +76,7 @@ impl Drop for MapperFlushAll {
     }
 }
 
+#[derive(Debug)]
 pub struct Mapper {
     p4: Unique<Table<Level4>>,
 }

src/arch/x86_64/paging/mod.rs

@@ -374,6 +374,7 @@ pub unsafe fn init_ap(
     init_tcb(cpu_id)
 }
 
+#[derive(Debug)]
 pub struct ActivePageTable {
     mapper: Mapper,
     locked: bool,