Update filecoin-cxx deps, mainly multihash

[cdesouza-chromium]

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.

Resolves brave/brave-browser#40947

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

[bridiver]

I'm not sure we still need this, looks like it's added in run_build_script.py now

[cdesouza-chromium]

Done

[cdesouza-chromium]

Removing this line seems to trigger the following failure

error: `CARGO_MANIFEST_DIR` env variable not set.
  --> ../../brave/third_party/rust/chromium_crates_io/vendor/multihash-0.18.1/src/multihash_impl.rs:11:34
   |
11 |   #[derive(Copy, Clone, Debug, Eq, Multihash, PartialEq)]
   |                                    ^^^^^^^^^ in this derive macro expansion
  --> ./../../brave/third_party/rust/chromium_crates_io/vendor/synstructure-0.12.6/src/macros.rs:94:9
  ::: ./../../brave/third_party/rust/chromium_crates_io/vendor/synstructure-0.12.6/src/macros.rs:96:40
   |
   = note: in this expansion of `#[derive(Multihash)]`

error: aborting due to 1 previous error

I'm not quite sure why run_build_script.py is failing to set CARGO_MANIFEST_DIR for multihash in particular, but this failure seems to be the reason this line has been here in the first place.