LibWeb+LibJS: Move some code around to make CSS/Parser parse faster

This makes it possible to include fewer full definitions of things,
which makes the file about 30% faster to compile.

Userland/Libraries/LibJS/CMakeLists.txt

@@ -23,6 +23,7 @@ set(SOURCES
     Contrib/Test262/IsHTMLDDA.cpp
     CyclicModule.cpp
     Heap/BlockAllocator.cpp
+    Heap/Cell.cpp
     Heap/CellAllocator.cpp
     Heap/Handle.cpp
     Heap/Heap.cpp

Userland/Libraries/LibJS/Heap/Cell.cpp

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2020-2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Heap/Cell.h>
+#include <LibJS/Heap/Heap.h>
+#include <LibJS/Runtime/Completion.h>
+#include <LibJS/Runtime/Value.h>
+
+namespace JS {
+
+ThrowCompletionOr<void> JS::Cell::initialize(JS::Realm&)
+{
+    return {};
+}
+
+void JS::Cell::Visitor::visit(JS::Value value)
+{
+    if (value.is_cell())
+        visit_impl(value.as_cell());
+}
+
+}

Userland/Libraries/LibJS/Heap/Cell.h

@@ -14,8 +14,6 @@
 #include <LibJS/Forward.h>
 #include <LibJS/Heap/GCPtr.h>
 #include <LibJS/Heap/Internals.h>
-#include <LibJS/Runtime/Completion.h>
-#include <LibJS/Runtime/Value.h>
 
 namespace JS {
 
@@ -33,7 +31,7 @@ class Cell {
     AK_MAKE_NONMOVABLE(Cell);
 
 public:
-    virtual ThrowCompletionOr<void> initialize(Realm&) { return {}; }
+    virtual ThrowCompletionOr<void> initialize(Realm&);
     virtual ~Cell() = default;
 
     bool is_marked() const { return m_mark; }
@@ -75,11 +73,7 @@ public:
             visit_impl(const_cast<RemoveConst<T>&>(*cell.ptr()));
         }
 
-        void visit(Value value)
-        {
-            if (value.is_cell())
-                visit_impl(value.as_cell());
-        }
+        void visit(Value value);
 
         // Allow explicitly ignoring a GC-allocated member in a visit_edges implementation instead
         // of just not using it.

Userland/Libraries/LibJS/Heap/Heap.h

@@ -21,6 +21,7 @@
 #include <LibJS/Heap/Handle.h>
 #include <LibJS/Heap/Internals.h>
 #include <LibJS/Heap/MarkedVector.h>
+#include <LibJS/Runtime/Completion.h>
 #include <LibJS/Runtime/WeakContainer.h>
 
 namespace JS {