[#14206,#14205] Cosmetics for of_int, of_nat

[volhovm]

Explain your changes:

• Both of_int and of_nat are now raising Invalid_argument instead of failwithf
• They are also renamed to of_int_exn and of_nat_exn

Explain how you tested your changes:

• Not tested explicitly

Checklist:

• Dependency versions are unchanged
  • Notify Velocity team if dependencies must change in CI
• Modified the current draft of release notes with details on what is completed or incomplete within this project
• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? List them

• Closes Proof_verified: of_int should raise Invalid_argument not a Failure. #14206 Proof_verified: of_nat should raise Invalid_argument not a Failure. #14205

[volhovm]

!ci-build-me

[volhovm]

Closing this PR since branch name with hashes # kills the CI. Will reopen with a different branch name.