Use optimised rlp hash writer in `aristo_compute`

`computeKey` is one of the places where we compute _the most_ hashes that would benefit from avoiding memory allocations (since they're numerous and small), and yet the new HashWriter is not in use there: https://github.com/status-im/nimbus-eth1/blob/232e7560e066810b67a09a193cd00bc4a4f9788d/execution_chain/db/aristo/aristo_compute.nim#L146