Investivate bit-packing the hashmap key in `ClassicCongestionControl::maybe_lost_packets`

[omansfeld]

Serious question: could we bit-pack the type and packet number into 64 bits? Would it help performance? Note that we only send and track packets from four of the types, so we probably could do that packing in the container. (I'd suggest that if you try, do that separately; in fact, it might even be a good idea to split the tracking piece from this work.)

Originally posted by @martinthomson in #3086 (comment)