AK: Remove OutputMemoryStream for DuplexMemoryStream.

OutputMemoryStream was originally a proxy for DuplexMemoryStream that
did not expose any reading API.

Now I need to add another class that is like OutputMemoryStream but only
for static buffers. My first idea was to make OutputMemoryStream do that
too, but I think it's much better to have a distinct class for that.

I originally wanted to call that class FixedOutputMemoryStream but that
name is really cumbersome and it's a bit unintuitive because
InputMemoryStream is already reading from a fixed buffer.

So let's just use DuplexMemoryStream instead of OutputMemoryStream for
any dynamic stuff and create a new OutputMemoryStream for static
buffers.

Shell/AST.cpp

@@ -1107,7 +1107,7 @@ void Execute::for_each_entry(RefPtr<Shell> shell, Function<IterationDecision(Ref
             } while (action == Continue);
 
             if (!stream.eof()) {
-                auto entry = ByteBuffer::create_uninitialized(stream.remaining());
+                auto entry = ByteBuffer::create_uninitialized(stream.size());
                 auto rc = stream.read_or_error(entry);
                 ASSERT(rc);
                 callback(create<StringValue>(String::copy(entry)));