Frame: Introduce `TryConvert` and `MaybeConvert` family of traits (#14297)

* Frame: Introduce TryConvert and TryConvertBack

* Update primitives/runtime/src/traits.rs

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Update primitives/runtime/src/traits.rs

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Update primitives/runtime/src/traits.rs

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Update primitives/runtime/src/traits.rs

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Update primitives/runtime/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update primitives/runtime/src/traits.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Add MaybeEquivalence

* Docs

---------

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>