* core: implement logic for tracking dag of possible pending changes
* core: move pending justifications dag to its own crate
* core: remove unnecessary clone bounds on dag
* core: request justifications in-order from the dag
* core: dag: rename changes variables to node
* core: dag: allow finalizing blocks not part of dag
* core: dag: track best finalized number
* core: dag: add more tests
* core: sync: clean up pending justifications dag
* core: dag: derive codec decode encode
* core: dag: better error support
* core: dag: add finalization guarded by predicate
* core: grandpa: track multiple authority set changes in dag
* core: dag: add pre-order iterator
* core: grandpa: request justifications on startup
* core: dag: rearrange order of definitions
* core: rename util/dag to util/fork_tree
* core: fork_tree: add docs
* core: fork_tree: add more tests
* core: fork_tree: fix issues found in tests
* core: grandpa: fix authorities tests
* core: grandpa: add docs for is_descendent_of
* core: sync: add docs for PendingJustifications
* core: sync: add test for justification requests across forks
* core: sync: don't resend import or finality notifications in tests
* core: grandpa: add test for importing multiple change blocks
* core: grandpa: fix logic for checking if a block enacts a change
* core: grandpa: fix authorities tests
* core: grandpa: move consensus_changes to its own submodule
* core: grandpa: move justification to its own submodule
* core: grandpa: move environment to its own submodule
* core: grandpa: move block import to its own submodule
André Silva