Approval Voting Database (#2162)

* skeleton

* skeleton aux-schema module

* start approval types

* start aux schema with aux store

* doc

* finish basic types

* start approval types

* doc

* finish basic types

* write out schema types

* add debug and codec impls to approval types

* add debug and codec impls to approval types

also add some key computation

* add debug and codec impls to approval types

* getters for block and candidate entries

* grumbles

* remove unused AssignmentId

* load_decode utility

* implement DB clearing

* function for adding new block entry to aux store

* start `canonicalize` implementation

* more skeleton

* finish implementing canonicalize

* tag TODO

* implement a test AuxStore

* add allow(unused)

* basic loading and deleting test

* block_entry test function

* add a test for `add_block_entry`

* ensure range is exclusive at end

* test clear()

* test that add_block sets children

* add a test for canonicalize

* Update node/core/approval-voting/src/aux_schema/mod.rs

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

* Update node/core/approval-voting/src/aux_schema/tests.rs

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

* Update node/core/approval-voting/src/aux_schema/mod.rs

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>