LibJS: Make Executable ref-counted and let instruction iterator co-own it

This ensures that the instruction stream pointed at by the instruction
iterator remains valid as long as the iterator exists.

Userland/Libraries/LibJS/Forward.h

@@ -302,7 +302,7 @@ class MarkedVector;
 
 namespace Bytecode {
 class BasicBlock;
-struct Executable;
+class Executable;
 class Generator;
 class Instruction;
 class Interpreter;