* update to Altair as of v1.1.0-alpha.7
* introduce Altair types into attestation pool
* avoid allocating/copying pubkeys excessively in get_next_sync_committee()
* proposed structure for hf1
* refactor datatypes.nim into datatypes/{base, phase0, hf1}.nim
* hf1 is Altair
* some syncing with alpha 2
* adjust epoch processing to disambiguate access to RewardFlags
* relocate StateData to stay consistent with meaning phase 0 StateData
* passes v1.1.0 alpha 5 SSZ consensus object tests
* Altair block header test fixtures work
* fix slash_validator() so that Altair attester slashings, proposer slashings, and voluntary exit textures work
* deposit operation Altair test fixtures work
* slot sanity and all but a couple epoch transition tests switched to Altair
* attestation Altair test fixtures work
* Altair block sanity test fixtures work
* add working altair sync committee tests
* improve workarounds for sum-types-across-modules Nim bug; incorporate SignedBeaconBlock root reconstuction to SSZ byte reader
* update some v1.1.0 alpha1 to alpha2
* remove unused getDepositMessage overload and move other out of datatypes/base
* bump nim-eth2-scenarios to download v1.1.0-alpha.2 test vectors
* construct object rather than result
