Make decoder state completely relocatable by avoiding inline pointer to mode
Currently, Opus Decoder state can be freely moved in memory in the same process, but it can't e.g. be copied as bytes to be sent across the network and set as a state for another decoder. But it almost can! The only thing preventing this use case is the mode pointer at the start of CELTDecoder. A clean decoder state on one machine has identical bytes to those of a clean state on another machine with the same architecture, except for possibly the bytes of the mode pointer - even when the mode itself is the same.
I'm not sure how to fix this in a way compatible with any custom mode, but it would be nice to be able to do this for the default mode, at least.