feat(l1): compute block hash once

[Arkenan]

Description

Use OnceCell to cache hash computations for a single block in a thread safe way. It also doesn't require the block to be mutable in order to update the cache, which is ideal.

On a mac M1:

• The first hash computation (for a default block) takes about 150-450 μs. Storing it in the OnceCell is about 10μs.
• The subsequent hash() calls that get from the cache take ~40ns (10k times better).

Follow-up from #962
closes #1014