Remove custody_bits and AttestationDataAndCustodyBit

[djrtwo]

NOTE: This was Merged. There is some sort of github bug that is showing this as Closed, but disregard that.

These two constructions weren't originally removed from v0.9.0 to attempt to not disrupt the status quo where unnecessary. As we dig deeper into the Phase 1 spec, the custody bit construction is altered from this original construction to be on a per block basis and the placeholder in phase 0 would eventually have to be removed come phase 1 anyway.

I suggest we remove this and release as a v0.9.1 along with some other minor changes in the queue

changelog:

• remove custody_bits from Attestation
• replace bit_0_indices and bit_1_indices in IndexedAttestation with a singular attesting_indices
• remove AttestationDataAndCustodyBit and replace the attestation signing as just over attestation.data
• use a bls_verify instead of a bls_verify_multiple in process_attestation
• minor adjustments to validator and fork choice wrt these changes
• update tests accordingly

[protolambda]

We should make a plan for testing of phase 1, as the old custody code is assumed to be there still. It looks good otherwise, no more unstable stubs in phase 0 is probably for the best. 👍