Serde DKG for BLS12-377

[gakonst]

Supersedes on #6. It fixes the merge conflicts and utilizes serde's serialize_with and deserialize_with to implement it for our wrapper structs cleanly.

[gakonst]

Let's wait on an answer for serde-rs/serde#1778 before merging

[gakonst]

We can safely opt-out of the atomic feature since we only use BitVec:

BitBox and BitVec are theoretically thread-safe regardless of this feature, as the borrow checker’s rules combine with the allocator regime to forbid shared-mutation faults with these types. However, as a conservative rule and to maintain consistency, these types also remove their thread-safety markers when atomic is disabled.

https://github.com/myrrlyn/bitvec#atomic