This PR contains a minor optimization in derivesha, by exposing the RLP
int-encoding and making use of it to write integers directly to a
buffer (an RLP integer is known to never require more than 9 bytes
total). rlp.AppendUint64 might be useful in other places too.

The code assumes, just as before, that the hasher (a trie) will copy the
key internally, which it does when doing keybytesToHex(key).

Co-authored-by: Felix Lange <fjl@twurst.com>