Rename math functions to strict_add/strict_sub (backport #2107)

[mergify]

Turns out the Rust standard library add the functions from #2098 under a different name. See rust-lang/rust#118260. Since this was not yet release, we can use those names too.

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This is an automatic backport of pull request #2107 done by Mergify.

[mergify]

Cherry-pick of 3b59561 has failed:

On branch mergify/bp/release/1.5/pr-2107
Your branch is up to date with 'origin/release/1.5'.

You are currently cherry-picking commit 3b59561d5.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   packages/std/src/math/uint128.rs
	modified:   packages/std/src/math/uint256.rs
	modified:   packages/std/src/math/uint512.rs
	modified:   packages/std/src/math/uint64.rs
	modified:   packages/std/src/timestamp.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally