Attestation changes + persistent committee changes (ethereum#1294)

* Minimal attestation simplification

* minor fix

* Make the tests pass

* Decrease `PLACEHOLDER`, Use `compute_epoch_of_shard_slot`

* Fix proposer signature name and use get_seed() to calculate current_shuffling_seed

* Fix linter error

* Add the WIP `test_is_valid_shard_block`

* Add `get_shard_block_attester_committee`

* Simplified committee selection

* Added some helpers and simplified

* Update specs/core/1_shard-data-chains.md

* Update 1_shard-data-chains.md

* Simplified switchover epochs, changed block structure, changed crosslink structure

* Update 1_shard-data-chains.md

* Moved balance dependency to proposer selection

* Update specs/core/1_shard-data-chains.md

Co-Authored-By: Danny Ryan <[email protected]>

* Update specs/core/1_shard-data-chains.md

Co-Authored-By: Danny Ryan <[email protected]>

* Update specs/core/1_shard-data-chains.md

Co-Authored-By: Danny Ryan <[email protected]>

* Update specs/core/1_shard-data-chains.md

Co-Authored-By: Danny Ryan <[email protected]>

* Update specs/core/1_shard-data-chains.md

Co-Authored-By: Danny Ryan <[email protected]>

* Update specs/core/1_shard-data-chains.md

Co-Authored-By: Danny Ryan <[email protected]>

* Update specs/core/1_shard-data-chains.md

Co-Authored-By: Danny Ryan <[email protected]>

* Update specs/core/1_shard-data-chains.md

* Fixed shard header flattening

* Update specs/core/1_shard-data-chains.md

* Minor fixes

* Update specs/core/1_shard-data-chains.md

* Update specs/core/1_shard-data-chains.md

Co-Authored-By: Hsiao-Wei Wang <[email protected]>

* cleanup testing and lint

* return none if not active validators in persistent committee

* only allow active validators as shard proposer

scripts/build_spec.py

@@ -48,11 +48,10 @@
 from eth2spec.utils.ssz.ssz_impl import (
     hash_tree_root,
     signing_root,
-    serialize,
-    is_zero,
+    is_empty,
 )
 from eth2spec.utils.ssz.ssz_typing import (
-    bit, boolean, Container, List, Vector, Bytes, uint64,
+    uint64, bit, boolean, Container, List, Vector, Bytes, BytesN,
     Bytes1, Bytes4, Bytes8, Bytes32, Bytes48, Bytes96, Bitlist, Bitvector,
 )
 from eth2spec.utils.bls import (