hashx: More arrays (redux), unsafe FixedCapacityVec
See commit messages for benchmark results.
Prompted by beth/arti!2. Branch is on top of !1529 (merged). The first commit is the same as in my hashx-bench-2
branch, MR !1539 (merged) .
The branch is a bit messy and certainly should be tidied up if this seems like a good way forward.
Edited by Ian Jackson