LibIPC: Avoid redundant copy of every tranferred IPC message

For every IPC message sent, we currently prepend the message size to the
IPC message buffer. This incurs the cost of copying the entire message
to its newly allocated position. Instead, reserve the bytes for the size
at the front of the buffer upon creation. Prevent dangerous access to
the buffer with specific public methods.

Userland/Libraries/LibIPC/Encoder.h

@@ -37,20 +37,20 @@ public:
 
     ErrorOr<void> extend_capacity(size_t capacity)
     {
-        return m_buffer.data.try_ensure_capacity(m_buffer.data.size() + capacity);
+        TRY(m_buffer.extend_data_capacity(capacity));
+        return {};
     }
 
     ErrorOr<void> append(u8 const* values, size_t count)
     {
-        TRY(extend_capacity(count));
-        m_buffer.data.unchecked_append(values, count);
+        TRY(m_buffer.append_data(values, count));
         return {};
     }
 
     ErrorOr<void> append_file_descriptor(int fd)
     {
-        auto auto_fd = TRY(adopt_nonnull_ref_or_enomem(new (nothrow) AutoCloseFileDescriptor(fd)));
-        return m_buffer.fds.try_append(move(auto_fd));
+        TRY(m_buffer.append_file_descriptor(fd));
+        return {};
     }
 
     ErrorOr<void> encode_size(size_t size);