Primitives for SASSAFRAS.

# ⚠️ WARNING ⚠️

The crate interfaces and structures are highly experimental and may be subject
to significant changes.

Depends on upstream experimental feature: `bandersnatch-experimental`.

These structs were mostly extracted from the main SASSAFRAS protocol PR: https://github.com/paritytech/polkadot-sdk/pull/1336.

Tracking issue: https://github.com/paritytech/polkadot-sdk/issues/41
