* primitives/beefy: move Payload to its own file

* primitives/beefy: add Payload tests

* primitives/beefy: add MmrRootProvider as custom BEEFY payload provider

* client/beefy: use generic BEEFY 'PayloadProvider'

* primitives/beefy: rename Payload::new to Payload::from_single_entry for clarity

* fix visibility

* fix cargo doc