Use uint64 for GeneralizedIndex

[etan-status]

To support adressing any nimbus-eth2 BeaconState member, 32-bit wide
indices are not enough. BeaconState requires 5 levels of depth;
VALIDATOR_REGISTRY_LIMIT for the validators array requires 40 levels
for the contents plus 2 levels for len and the root. Another 4 levels
are required for Validator. This means that 52-bit indices are needed.
Extending to uint64 should provide enough leeway without requiring
further implementation adjustments.

[etan-status]

I think that the internal mergedDataHash etc which operate on int64 do not need adjustment, because they only operate on sub-trees where 1.GeneralizedIndex through 3.GeneralizedIndex (length) are mapped to array element 0, and 4.GeneralizedIndex is mapped to element 1 (i.e., two bits are stripped). So, even if a full-width uint64 index was hypothetically used, this should still work.

[tersec]

nimbus-eth2 doesn't support more than int32-allowed validatorindices, regardless. That's arguably an unfortunate implementation detail, but also a bottleneck regardless of this change.

[etan-status]

Wait for #19 before merging this.

[etan-status]

Wait for #16 before merging this.