LibJS: Add placeholder bytecode block sealing mechanism

After compiling bytecode, we should mark the memory read-only.
This currently does not work because it breaks instruction destruction.

I'm adding this anyway with a FIXME so we don't forget about it. :^)

Userland/Libraries/LibJS/Bytecode/Block.cpp

@@ -36,6 +36,13 @@ Block::~Block()
     }
 }
 
+void Block::seal()
+{
+    // FIXME: mprotect the instruction stream as PROT_READ
+    // This is currently not possible because instructions can have destructors (that clean up strings)
+    // Instructions should instead be destructor-less and refer to strings in a string table on the Bytecode::Block.
+}
+
 void Block::dump() const
 {
     Bytecode::InstructionStreamIterator it(instruction_stream());

Userland/Libraries/LibJS/Bytecode/Block.h

@@ -42,6 +42,7 @@ public:
     static NonnullOwnPtr<Block> create();
     ~Block();
 
+    void seal();
     void dump() const;
     ReadonlyBytes instruction_stream() const { return ReadonlyBytes { m_buffer, m_buffer_size }; }
 

Userland/Libraries/LibJS/Bytecode/Generator.cpp

@@ -28,6 +28,7 @@ OwnPtr<Block> Generator::generate(ASTNode const& node)
     Generator generator;
     [[maybe_unused]] auto dummy = node.generate_bytecode(generator);
     generator.m_block->set_register_count({}, generator.m_next_register);
+    generator.m_block->seal();
     return move(generator.m_block);
 }