refactor: apply clippy::needless_splitn

[shulaoda]

Summary

• Replaces splitn(2, ..).next() with split(..).next() in
  src/overflow.rs and src/visitor.rs.
• Replaces rsplitn(2, ..).next() with rsplit(..).next() in
  src/utils.rs.
• Addresses clippy::needless_splitn, run as a single-rule pass:
  cargo clippy -- -A clippy::all -W clippy::needless_splitn.

When only the first element is consumed, the splitn(2, ..) upper bound
is redundant — split / rsplit are equally lazy iterators. Pure
refactor, no behavior changes intended. Touches 3 files.

[ytmimi]

Going to close until we decide on which clippy lints we want to enforce in the codebase.