Implement add for Uint* more consistently (backport #2092)

[mergify]

• Streamline implementation for references by using forward_ref_binop!
• use better error panic message for overflows for Uint64/Uint512 (before "called Option::unwrap() on a None value")

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  This is an automatic backport of pull request Implement add for Uint* more consistently #2092 done by Mergify.

[mergify]

Cherry-pick of 80b6ba7 has failed:

On branch mergify/bp/release/1.5/pr-2092
Your branch is up to date with 'origin/release/1.5'.

You are currently cherry-picking commit 80b6ba7b3.
  (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

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

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