Fix underflows from #1017

[djrtwo]

prevent underflows in generate_seed, get_randao_mix, and get_active_index_root.

Note that for epochs less than MIN_SEED_LOOKAHEAD we use ZERO_HASH rather than just pulling from get_randao_mix(state, GENESIS_EPOCH) because the randao_mix at GENESIS_EPOCH is not static and would thus break lookahead.

[hwwhww]

Good catch on randao_mix at GENESIS_EPOCH! 👍

Some epoch/get_current_epoch(state) confusions in my comments.

[JustinDrake]

A few initial comments:

1. Lines such as min_epoch = get_current_epoch(state) + ACTIVATION_EXIT_DELAY - LATEST_ACTIVE_INDEX_ROOTS_LENGTH + 1 if epoch > LATEST_ACTIVE_INDEX_ROOTS_LENGTH - ACTIVATION_EXIT_DELAY else GENESIS_EPOCH do not fit within the 120 character limit.
2. Readability for these helper functions has been significantly reduced.
3. Looking forward to removing the untriggerable assert in get_block_root_at_slot, get_randao_mix, get_active_index_root, get_shuffled_index, integer_squareroot, get_matching_source_attestations 😂Keen to find alternative testing solutions that do not pollute the state transition spec (in a similar vein to Monkeypatch the minus operator #1029).

[djrtwo]

@JustinDrake I'm okay taking the asserts out as they are untriggerable in the context of the state transition function. I added notes in the comment on the expected range for implementers whom I know utilize in other contexts