LibGC: Make GC::Ptr, GC::Ref, and GC::Root formattable

[AtkinsSJ]

This is a copy of the Formatters we have for AK's pointer types.

It was annoying me having to type .ptr() whenever I wanted to dbgln() something.

[AtkinsSJ]

Changes: Added GC::Root too.

[AtkinsSJ]

After the discord discussion where we couldn't actually decide how pointer formatting should work, and that not being able to format GC pointers keeps bothering me, I'm going to go ahead and merge this for now. (It was good enough for the AK pointers, so it should be good enough.) If and when we decide what they should all do, we can change them then.