## Issue Addressed
Closes#2354
## Proposed Changes
Add a `minify` method to `slashing_protection::Interchange` that keeps only the maximum-epoch attestation and maximum-slot block for each validator. Specifically, `minify` constructs "synthetic" attestations (with no `signing_root`) containing the maximum source epoch _and_ the maximum target epoch from the input. This is equivalent to the `minify_synth` algorithm that I've formally verified in this repository:
https://github.com/michaelsproul/slashing-proofs
## Additional Info
Includes the JSON loading optimisation from #2347
Michael Sproul
6583ce325b