LibJS: Inline Cell::Visitor::visit() functions

This allows the calls to MarkingVisitor::visit_impl() during GC to be
devirtualized in Heap::mark_live_cells().

Userland/Libraries/LibJS/CMakeLists.txt

@@ -2,7 +2,6 @@ set(SOURCES
     AST.cpp
     Console.cpp
     Heap/Allocator.cpp
-    Heap/Cell.cpp
     Heap/Handle.cpp
     Heap/HeapBlock.cpp
     Heap/Heap.cpp