Address Clippy warnings in tests

Instruct Clippy to also check tests

Use new format strings

Use Rust 2021

Add `Session::insert` to insert changes without references

Add `Chronofold::empty` to create chronofolds with empty logs

Add `flake.{nix,lock}` for Nix Flakes support

Bump version

Bump version

Merge branch 'fix-deletes'

Store deletion markers in the log

Apart from fixing concurrent insertions referencing deletions, this simplifies
and speeds up a couple of things:

1. Ordering of deletions in the causal tree is no special case anymore
2. Remove need for "look ahead" for deletions when iterating over elements
3. It will be easier to construct previous states in future, as each change now
   contains both the indexes for its "birth" and "death".

Add (failing) test for concurrent inserts referencing deletions

Implement `formatted_log` for nicer debug output

Address new Clippy warnings

Fix type errors in benchmarks

Bump version

Make `log_index` and `timestamp` public

Add `ChronofoldError::FutureTimestamp`

Revamp `Op`: remove `Change`, add `OpPayload`

This allows for having ops contain different types as the local chronofold, as
long as they are convertible via the new `FromLocalValue` and `IntoLocalValue`
traits.

Store root entry in the log