LibJS+Clients: Add JS::VM object, separate Heap from Interpreter

Taking a big step towards a world of multiple global object, this patch
adds a new JS::VM object that houses the JS::Heap.

This means that the Heap moves out of Interpreter, and the same Heap
can now be used by multiple Interpreters, and can also outlive them.

The VM keeps a stack of Interpreter pointers. We push/pop on this
stack when entering/exiting execution with a given Interpreter.
This allows us to make this change without disturbing too much of
the existing code.

There is still a 1-to-1 relationship between Interpreter and the
global object. This will change in the future.

Ultimately, the goal here is to make Interpreter a transient object
that only needs to exist while you execute some code. Getting there
will take a lot more work though. :^)

Note that in LibWeb, the global JS::VM is called main_thread_vm(),
to distinguish it from future worker VM's.

Libraries/LibJS/Runtime/VM.h

@@ -0,0 +1,63 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/RefCounted.h>
+#include <LibJS/Heap/Heap.h>
+
+namespace JS {
+
+class VM : public RefCounted<VM> {
+public:
+    static NonnullRefPtr<VM> create();
+    ~VM();
+
+    Heap& heap() { return m_heap; }
+    const Heap& heap() const { return m_heap; }
+
+    Interpreter& interpreter();
+    Interpreter* interpreter_if_exists();
+
+    void push_interpreter(Interpreter&);
+    void pop_interpreter(Interpreter&);
+
+    class InterpreterScope {
+    public:
+        InterpreterScope(Interpreter&);
+        ~InterpreterScope();
+    private:
+        Interpreter& m_interpreter;
+    };
+
+private:
+    VM();
+
+    Heap m_heap;
+    Vector<Interpreter*> m_interpreters;
+};
+
+}