Update `filecoin-cxx` deps, mainly `multihash` This change is a first pass in getting the dependecies for `filecoin-cxx` updated, to eventually be able to drop the unmaintained crate `proc-macro-error`. This change updates `multihash` to `v0.16.3`->`v0.18.1`. In a follow up PR, the goal will be to deploy `v0.19.1`, which drops the unwanted crate as a transient dependency. --------- Co-authored-by: Ralph Giles <rgiles@brave.com> Co-authored-by: Claudio DeSouza <cdesouza@brave.com>