feat: add bitlists and bitvectors natively in SSZ nif library

[Arkenan]

Changes

• Remove the length from the bitlist internal representation.
• Add encode/decode functions to attestation and pending attestation containers, so that the old library can handle bitlists.
• Add encode/decode functions to beacon state and sync aggregate so the old library can handle bitvectors.
• In all points integrating with bitlists and bitvectors (mostly operations and epoch processing), remove the creation and serialization of bitlists and bitvectors, as they are now part of the encoding and decoding process.

Closes #784

[MegaRedHand]

LGTM