Stabilize const {integer}::from_str_radix i.e. const_int_from_str

[Skgland]

This PR stabilizes the feature const_int_from_str.

• ACP Issue: Make FromStr on integers const libs-team#74
• Implementation PR: Make {integer}::from_str_radix constant #99322
• Part of Tracking Issue: Tracking issue for FromStr trait usage in const fn #59133

API Change Diff:

impl {integer} {
- pub       fn from_str_radix(src: &str, radix: u32) -> Result<Self, ParseIntError>;
+ pub const fn from_str_radix(src: &str, radix: u32) -> Result<Self, ParseIntError>;
}

impl ParseIntError {
- pub       fn kind(&self) -> &IntErrorKind;
+ pub const fn kind(&self) -> &IntErrorKind;
}

This makes it easier to parse integers at compile-time, e.g.
the example from the Tracking Issue:

env!("SOMETHING").parse::<usize>().unwrap()

could now be achived with

match usize::from_str_radix(env!("SOMETHING"), 10) {
  Ok(val) => val,
  Err(err) => panic!("Invalid value for SOMETHING environment variable."),
}

rather than having to depend on a library that implements or manually implement the parsing at compile-time.

---

Checklist based on Libs Stabilization Guide - When there's const involved

I am treating this as a partial stabilization as it shares a tracking issue (and is rather small), so directly opening the partial stabilization PR for the subset (feature const_int_from_str) being stabilized.

• ping Constant Evaluation WG
• #[allow_internal_unstable] used for access to internal can_not_overflow.
  • suggestions for alternatives are welcome
  • This is limited to can_not_overflow, which is an internal const fn. And it should be Ok as that function could just be inlined to remove the internally unstable call.
• no unsafe involved
• usage of intrinsic::const_eval_select Tracking Issue for const_eval_select #124625 in from_str_radix_assert to change the error message between compile-time and run-time
• rust-labg/libs-api FCP

[rustbot]

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[scottmcm]

Looks like there hasn't been an FCP for this, so I have to flip it over to
r? libs-api