This is an optimisation that allows for faster hashing of a public key,
however it adds a penalty to SSZ encoding because we need to go
decompressed -> PublicKey -> compressed.
The spec presently uses compressed bytes to store public keys, however
I'm hoping it will change.
- Adds a new-type wrapper for `AggregatePublicKey`, just like all the
other types.
- Adds the `verify_multiple` method to the `AggregateSignature` newtype,
as was introduced in a recent version of signature-schemes.
