WIP: Modernize HltPackedData{Writer,Decoder}
- make HltPackedDataDecoder thread safe
- move code and data common to Writer and Decoder into HltPackedData.h
- amalgamate decoder and writer headers into their .cpp files
- introduce LHCb::Hlt::PackedData namespace
- prefer std::string_view over const std::string&
Edited by Gerhard Raven