Rename BytesN to ByteVector, rename Bytes to ByteList

[protolambda]

People reading the pyspec SSZ util code got confused by Bytes vs BytesN. So this PR changes it to a more verbose but also more clear ByteList and ByteVector. No functional changes.

Fixes #1479

[djrtwo]

I support this

[JustinDrake]

ByteList, ByteVector, Bitlist, Bitvector

I also support this but I guess we need consistent capitalisation. Could rename Bitlist to BitList.

[protolambda]

Agree with Justin, but there's something to mixed capitalisation too: ByteVector[N] is exactly a Vector[Byte, N], whereas Bitvector is not a Vector[Bit, N]: it's like a bitfield instead, a full word without stops. Same thing for the list variant.

[djrtwo]

Ah, you're right @protolambda. Unfortunately the subtlety in the difference in capitalization doesn't really send much of a signal to a dev imo.

That said, I think I lean toward keeping as is to reduce overhead in changing implementations and capturing the minor difference

[djrtwo]

Ah, maybe should have moved to v09x. I suppose no need to get this out into a release soon