Explicitly parent_root check in state transition

[JustinDrake]

No description provided.

[hwwhww]

1. But hasn't it been checked in the "processing block conditions"?
   https://github.com/ethereum/eth2.0-specs/blob/dev/specs/core/0_beacon-chain.md#beacon-chain-processing
   • The parent block with root block.parent_root has been processed and accepted.
2. For nodes (not only validators), it also implied that they'd switch the local view of BeaconState to the view of block parent_root (parent_block.state_root) before applying state transition.

[JustinDrake]

I see the sections "Beacon chain processing" and "Beacon chain state transition function" as serving two different functions.

The first section involves subjectivity (time, observed blocks, observed offchain attestations, ...) whereas the second is a purely objective state transition function. The parent_root check is an objective check part of the state transition function. It doesn't hurt to mention parent_root in the first section to help the presentation.

[JustinDrake]

Closing in favour of #649.