saturating arithmetic methods don’t work in `const fn`

[sbillig]

Keep saturating methods evaluable during CTFE

Replacing the per-type Fe implementations with calls to __saturating_* makes every const/array-length/other CTFE use of saturating_add/sub/mul hit an extern. CtfeInterpreter::eval_extern_const_fn only handles __as_bytes and __keccak256 (crates/hir/src/analysis/ty/ctfe.rs:1414), so expressions like const N: u8 = 255u8.saturating_add(1); now regress from being foldable to ConstEvalUnsupported. The old bodies in num.fe were plain Fe code, so this is a new user-visible loss of functionality in compile-time contexts.

Originally posted by @chatgpt-codex-connector[bot] in #1347 (comment)