explicitly represent symbol death as a historical event

[tayloraswift]

right now, Biome has no concept of symbol death, this just shows up as a gap in its history. this is because Biome does not have a good awareness right now of symbol inflows/outflows coming from each package snapshot.

this is also somewhat wasteful, because we need to store a double-bounded range of versions for each node in a symbol’s history. storing explicit symbol death events would reduce the amount of symbol metadata stored per event.

child of: #46