Move BitList and BitVector to ethereum_ssz

[macladson]

In preparation of EIP-7495, it will be very useful to have a fully functional BitVector implementation in this crate, so we don't need to depend on ssz_types, avoiding a circular dependency.

This moves the two types previously defined in https://github.com/sigp/ssz_types to ethereum_ssz/ssz. This will allow us to utilize it within ssz_derive.

This PR should be paired with an equivalent PR in ssz_types which removes the types and instead re-exports them from ethereum_ssz. It should also be paired with a PR in tree_hash which implements TreeHash for BitVector and BitList.

Related PRs:

• Remove Bitfield ssz_types#28
• Implement TreeHash for bitfield tree_hash#18

[codecov]

Codecov Report

Attention: Patch coverage is 71.68142% with 64 lines in your changes missing coverage. Please review.

Project coverage is 77.48%. Comparing base (c28ae8c) to head (91b5130).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
ssz/src/bitfield.rs	71.68%	64 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
- Coverage   80.16%   77.48%   -2.69%     
==========================================
  Files          11       12       +1     
  Lines         489      715     +226     
==========================================
+ Hits          392      554     +162     
- Misses         97      161      +64     

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[macladson]

I've hidden these types in the docs which I believe means the current documentation should remain unchanged.

Since we are re-exporting the Bitfield types in ssz_types the documentation for them will still exist in that crate. Like it does now.

There might be an argument for moving the documentation across (or duplicating it), but I'm loosely in favour of this approach for now.

[michaelsproul]

LGTM. The diff from ssz_types is minimal now that resize has been added.