LibPDF: Create basic object structure

This commit is the start of LibPDF, and introduces some basic structure
objects. This emulates LibJS's Value structure, where Value is a simple
class that can contain a pointer to a more complex Object class with
more data. All of the basic PDF objects have a representation.

Userland/Libraries/LibPDF/Value.cpp

@@ -0,0 +1,60 @@
+/*
+ * Copyright (c) 2021, Matthew Olsson <mattco@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibPDF/Object.h>
+#include <LibPDF/Value.h>
+
+namespace PDF {
+
+Value::~Value()
+{
+    if (is_object())
+        m_as_object->unref();
+}
+
+Value& Value::operator=(const Value& other)
+{
+    m_type = other.m_type;
+    switch (m_type) {
+    case Type::Null:
+        break;
+    case Type::Bool:
+        m_as_bool = other.m_as_bool;
+        break;
+    case Type::Int:
+        m_as_int = other.m_as_int;
+        break;
+    case Type::Float:
+        m_as_float = other.m_as_float;
+        break;
+    case Type::Object:
+        m_as_object = other.m_as_object;
+        if (m_as_object)
+            m_as_object->ref();
+        break;
+    }
+    return *this;
+}
+
+String Value::to_string(int indent) const
+{
+    switch (m_type) {
+    case Type::Null:
+        return "null";
+    case Type::Bool:
+        return as_bool() ? "true" : "false";
+    case Type::Int:
+        return String::number(as_int());
+    case Type::Float:
+        return String::number(as_float());
+    case Type::Object:
+        return as_object()->to_string(indent);
+    }
+
+    VERIFY_NOT_REACHED();
+}
+
+}