perf: Misc perf improvements

[cchudant]

Hi,
I have been working a bit on perf, and following discussions with @AurelienFT, I think it's better to split it into separate prs

This one does not change any of the public api and I think should be easy and uncontroversial to merge

in total, this gives about a 20% improvement on the insert 40k random keys into a contract.
The benches use HashMap db and they are handcrafted, they may not accurately translate to these numbers in the real world - I am simply just following what the vtune profiler tells me is bad

For the other prs: the main one I really want adds a bonsai trie many_insert, which does GETs and tree traversal in parallel but I've had some complications as Rocksdb transaction object isnt Sync, which means I can't just reuse the existing transactional state merge api without having to change it quite a bit
I think I have come up with a better way though, so I'll drop the PR asap and we'll see then if it's nice enough

The other one is about switching all keys to smallvec storage, it kind of changes the api a bit at some places

[AurelienFT]

Thank you for the very good work on this PR that improve readability and performances.