feat/view: assure heads in a view are sorted (#2493)

* feat/view: assure heads in a view are sorted

Allows O(n) comparisons, adds an alternate equiv relation
which takes O(n^2) for integrity verification.

Ref #2133

* revert: remove custom PartialEq impl, there are no duplicates

* fix: do not sort the live_heads, that alters the local view

* refactor/view: heads should not be public

* chore/spellcheck: add unfinalized

* fix/view: add missing len() and is_empty() fns

* quirk

* vec is not view

* Update node/network/approval-distribution/src/tests.rs

Co-authored-by: Andronik Ordian <[email protected]>

* Update node/network/bridge/src/lib.rs

Co-authored-by: Andronik Ordian <[email protected]>

* Update node/network/protocol/src/lib.rs

Co-authored-by: Andronik Ordian <[email protected]>

* fixup comment

* fix botched test

Co-authored-by: Andronik Ordian <[email protected]>