Implement from_be_bytes/from_le_bytes for Uint64 and Uint128 (backport #2351)

[mergify]

Closes #2269

We could use a similar macro for the larger types based on bnum, but that would be a lot of extra work that does not provide an immediate benefit. I'd do that later maybe but in a separate PR.

---

This is an automatic backport of pull request #2351 done by Mergify.

[mergify]

Cherry-pick of 73dd089 has failed:

On branch mergify/bp/release/2.2/pr-2351
Your branch is ahead of 'origin/release/2.2' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 73dd0892.
  (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/conversion.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   packages/std/src/math/int128.rs
	both modified:   packages/std/src/math/int64.rs

Cherry-pick of 23de2fc has failed:

On branch mergify/bp/release/2.2/pr-2351
Your branch is ahead of 'origin/release/2.2' by 2 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 23de2fcd.
  (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)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md
	both modified:   packages/std/src/math/uint128.rs
	both modified:   packages/std/src/math/uint64.rs

no changes added to commit (use "git add" and/or "git commit -a")

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