* remove near-duplicate code paths: process_slot(), process_slots(), and state_transition() for BeaconState are now wrappers around the HashedBeaconState versions
* convert tests/test_state_transition.nim to use HashedBeaconState
* convert mocking infrastructure and spec_block/epoch_processing tests to use HashedBeaconBlock, and remove thus unused process_slot*(state: var BeaconState)