LibJS: Start fleshing out a bytecode for the JavaScript engine :^)

This patch begins the work of implementing JavaScript execution in a
bytecode VM instead of an AST tree-walk interpreter.

It's probably quite naive, but we have to start somewhere.

The basic idea is that you call Bytecode::Generator::generate() on an
AST node and it hands you back a Bytecode::Block filled with
instructions that can then be interpreted by a Bytecode::Interpreter.

This first version only implements two instructions: Load and Add. :^)

Each bytecode block has infinity registers, and the interpreter resizes
its register file to fit the block being executed.

Two new `js` options are added in this patch as well:

`-d` will dump the generated bytecode
`-b` will execute the generated bytecode

Note that unless `-d` and/or `-b` are specified, none of the bytecode
related stuff in LibJS runs at all. This is implemented in parallel
with the existing AST interpreter. :^)

Userland/Libraries/LibJS/CMakeLists.txt

@@ -1,25 +1,30 @@
 set(SOURCES
-    AST.cpp
-    Console.cpp
-    Heap/CellAllocator.cpp
-    Heap/BlockAllocator.cpp
-    Heap/Handle.cpp
-    Heap/HeapBlock.cpp
-    Heap/Heap.cpp
-    Interpreter.cpp
-    Lexer.cpp
-    MarkupGenerator.cpp
-    Parser.cpp
-    Runtime/Array.cpp
-    Runtime/ArrayBuffer.cpp
-    Runtime/ArrayBufferConstructor.cpp
-    Runtime/ArrayBufferPrototype.cpp
-    Runtime/ArrayConstructor.cpp
-    Runtime/ArrayIterator.cpp
-    Runtime/ArrayIteratorPrototype.cpp
-    Runtime/ArrayPrototype.cpp
-    Runtime/BigInt.cpp
-    Runtime/BigIntConstructor.cpp
+        AST.cpp
+        Bytecode/Block.cpp
+        Bytecode/Generator.cpp
+        Bytecode/Instruction.cpp
+        Bytecode/Interpreter.cpp
+        Bytecode/Op.cpp
+        Console.cpp
+        Heap/CellAllocator.cpp
+        Heap/BlockAllocator.cpp
+        Heap/Handle.cpp
+        Heap/HeapBlock.cpp
+        Heap/Heap.cpp
+        Interpreter.cpp
+        Lexer.cpp
+        MarkupGenerator.cpp
+        Parser.cpp
+        Runtime/Array.cpp
+        Runtime/ArrayBuffer.cpp
+        Runtime/ArrayBufferConstructor.cpp
+        Runtime/ArrayBufferPrototype.cpp
+        Runtime/ArrayConstructor.cpp
+        Runtime/ArrayIterator.cpp
+        Runtime/ArrayIteratorPrototype.cpp
+        Runtime/ArrayPrototype.cpp
+        Runtime/BigInt.cpp
+        Runtime/BigIntConstructor.cpp
     Runtime/BigIntObject.cpp
     Runtime/BigIntPrototype.cpp
     Runtime/BooleanConstructor.cpp