LibGC: Rename MarkedVector => RootVector

Let's try to make it a bit more clear that this is a Vector of GC roots.

Libraries/LibGC/RootVector.cpp

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <andreas@ladybird.org>
+ * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibGC/Heap.h>
+#include <LibGC/RootVector.h>
+
+namespace GC {
+
+RootVectorBase::RootVectorBase(Heap& heap)
+    : m_heap(&heap)
+{
+    m_heap->did_create_marked_vector({}, *this);
+}
+
+RootVectorBase::~RootVectorBase()
+{
+    m_heap->did_destroy_marked_vector({}, *this);
+}
+
+RootVectorBase& RootVectorBase::operator=(RootVectorBase const& other)
+{
+    if (m_heap != other.m_heap) {
+        m_heap = other.m_heap;
+
+        // NOTE: IntrusiveList will remove this RootVectorBase from the old heap it was part of.
+        m_heap->did_create_marked_vector({}, *this);
+    }
+
+    return *this;
+}
+
+}