Define Bitlist/Bitvector serialization using bytes, not bigints

[dankrad]

This does not change any logic and the serialization and Merkleization is exactly as before, but now defined using byte operations instead of bigints.

[hwwhww]

@dankrad
also need to update the SSZ spec doc as well. :)

[protolambda]

It looked like this PR was stuck, so I updated it.

• reduced the code duplication with an .as_bytes() in the pyspec implementation
• updated the ssz spec doc to also directly go from bits to bytes (instead of intermediate as_integer)

@dankrad a review back is welcome :)

[protolambda]

Also: the merkleization part is cleaned up in #1292, along with other merkleization wording/implementation fixes