str4d

No further experimentation happened in this PR, and we have since started migrating away from a plain FFI towards using the `cxx` crate. So it's even more unlikely that we'd...

[Rebased on `master`](https://github.com/zcash/zcash/compare/1d6bebfd1d1a047b06c57b835502a922100014c5..b7b0e8eaafe41ea43cd0a767546b7913ae2f9e09) and addressed my two previous comments. Running CI to see what the test failure is.

The assertion failure is being caused by the defensive assertion added to `CChain::FindFork` in #5862. Now I need to figure out why the changes in this PR (which ostensibly are...

5cf6ebd16983fe78831cecda6451759862899623 is the problematic commit. This likely means that it is backported incorrectly (which is reasonable, it's a complex backport and our code isn't in the same state as the...

[Rebased](https://github.com/zcash/zcash/compare/5f55b6913cf2114450a9f7785ed27c70223feb70..9ad6c7a975e4f3c575ce57e4b8d4287609c8957d) on current `master` to fix merge conflicts.

[Rebased on current `master`](https://github.com/zcash/zcash/compare/9ad6c7a975e4f3c575ce57e4b8d4287609c8957d..2d666fdad2d8ff6a46386db61b010ac6e7b34bd0) to fix merge conflicts.

[Rebased on `master`](https://github.com/zcash/zcash/compare/2d666fdad2d8ff6a46386db61b010ac6e7b34bd0..d6a5794cdcb5ee7caf896c0e2b31aa195c84dec5) now that #6377 has been merged.

[Rebased on `master`](https://github.com/zcash/zcash/compare/d6a5794cdcb5ee7caf896c0e2b31aa195c84dec5..b1dc94249c16756dcd51f6fd9a252152995f1624) to fix merge conflicts.

[Rebased on current `master`](https://github.com/zcash/zcash/compare/e3c0417559ada3a14d7f5e5fab24a1fcf005f782..35ba628f56d6b3deba88fc250dc05fb21e2c384f), which required making a few changes to the PR: - We now set `-flto=thin` in `CFLAGS` and `LDFLAGS` instead of appending to `CC`, as the latter...

[Rebased on current `master`](https://github.com/zcash/zcash/compare/35ba628f56d6b3deba88fc250dc05fb21e2c384f..42a7db2cf5257abc013c63442652d5b638a3c534) to fix merge conflicts.