perf(sparse): move-encode account RLP into trie to avoid clone

[ANtutov]

Encode account RLP into a fresh Vec and move it into the trie instead of cloning a reusable buffer. This removes one allocation and memcpy per account update. The persistent buffer in SparseStateTrie was removed since the trie must own the bytes; keeping it yielded no benefit and forced a clone. Mirrored the same change in the stateless path for consistency. Only allocation and copy reductions.

[mediocregopher]

This removes one allocation and memcpy per account update
Can you explain how so? The clone was previously an allocation, now you're allocating on the Vec::with_capacity instead. Functionally this looks equivalent to me, just a bit cleaner because no need for a reusable buffer. Just to check I'm understanding right...