LibPDF: Implement loading compressed objects from object streams

Now, whenever the xref table points to a compressed object,
parse_object_with_index will look it up in the corresponding object
stream as if it were a regular object.

With this, our parser gains the bare minimum support for xref streams.

Userland/Libraries/LibPDF/Parser.h

@@ -35,6 +35,9 @@ public:
 
     String parse_comment();
 
+    void move_by(size_t count) { m_reader.move_by(count); }
+    void move_to(size_t offset) { m_reader.move_to(offset); }
+
     PDFErrorOr<Value> parse_value();
     PDFErrorOr<Value> parse_possible_indirect_value_or_ref();
     PDFErrorOr<NonnullRefPtr<IndirectValue>> parse_indirect_value(u32 index, u32 generation);