Better indexing and sorting

[breuleux]

• Update to serieux 0.3.8
• Use FileBacked from serieux as a backing for FileCollection
• Replace Finder by Index, make it more generic
• Sort papers by latest venue

[satyaog]

Would the following be useful to avoid unexpected replacement of an entry?

Suggested change

	def index(self, entry: T):
	for name, fn in self.indexers.items():
	idx = self.indexes[name]
	for value in fn(entry):
	idx[value] = entry
	def index(self, entry: T):
	for name, fn in self.indexers.items():
	idx = self.indexes[name]
	for value in fn(entry):
	assert value not in idx or idx[value] == entry
	idx[value] = entry

[breuleux]

Collisions aren't necessarily a big deal here, they might happen for titles, but that's not really going to break anything.