* 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()
* 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