Update bridge pallet to work with the (almost) lastest master (#4672)

* Update decl_error usage

* WIP: Update error handling to use DispatchResult

* Get module compiling with new error handling

* Make tests compile again

Main change was updating the usage of InMemoryBackend

* Move `sp-state-machine` into dev-dependencies

* Bump dependencies to v2.0.0

* Remove some stray comments

* Appy code review suggestion