Switch to libssz

[jsign]

This PR switches from Ligthouse SSZ library to Lambdaclass new SSZ library.

This has many benefits:

1. libssz has native no_std support, so it allows us to now run Airbender.
2. From the measurements I've done with Zisk cost calculator, the hash tree root for the output commitment is 2x faster. Technically, this should result in an overall proving speedup of ~5% (previous hash tree root influence in total proving time was ~10%).
3. Much less indirect dependencies, and also remove one ethereum_hashing patch I had to do, so less complexity/forks.

Fixes #8

[han0110]

LGTM! Just a nit on naming consistency, it's good to keep as is.