libssz v0.3.0

Upgrades to match the latest version of the consensus-specs (v1.7.0-alpha.13).

Breaking Changes

• Progressive merkleization roots changed (EIP-7916). Affects ProgressiveList, ProgressiveBitlist, #[ssz(progressive_container)].
• Unknown #[ssz(...)] keys are now compile errors, not silently ignored.
• Progressive containers capped at 256 fields, per spec.

Added

• #[ssz(progressive_container, active_fields = [1, 0, 1])] for containers with inactive fields. Defaults to all-active, so existing types are unchanged.

Performance

• merkleize elides all-zero subtrees and reuses buffers instead of allocating per layer.