They should have 32 bits of padding after the error code now.

Popping objects from the buffer is still not implemented.

IPC: Use the correct size when pushing raw data to the command buffer and fixed pushing domain objects.

Domain object ids are always stored immediately after the raw data.

Reduces the necessary allocation to max(in_handles, out_handles) rather
than (in_handles + out_handles).

These use the context functions to create and look-up handles for the
user.