[Chronik] Add Eq derives for BlocksError and SerError

Summary:
Rust 1.63 added derive_partial_eq_without_eq to clippy, which gives us a lint that when we derive PartialEq, we should also derive Eq when we can.

Eq requires reflexivity, i.e. a == a. PartialEq is for the case where this is not necessarily true, like for NaN for floats. Since we have reflexivity for these errors, the lint is a true positive and this diff fixes it.

Test Plan:

1. Upgrade Rust to 1.63 (rustup install 1.63 or rustup update)
2. Verify the lint is triggered (cargo clippy in /)
3. (optional) Downgrade Rust to 1.61 (rustup install 1.61)

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12018