Use optimised rlp hash writer in aristo_compute

[arnetheduck]

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:

nimbus-eth1/execution_chain/db/aristo/aristo_compute.nim

Line 146 in 232e756

var writer = initRlpWriter()

[arnetheduck]

More here: https://github.com/status-im/nim-eth/blob/37bf6db28b81e3cd45a101457bc6909d9081c4b4/eth/common/transactions_rlp.nim#L238

basically, need to grep the codebase for any rlp interactions that we can get rid of