Add `isqrt` to `NonZero<uN>`
Implements [#70887 (comment)](https://github.com/rust-lang/rust/issues/116226#issuecomment-2144225174), with the following signature:
```rust
impl NonZero<uN> {
const fn isqrt(self) -> Self;
}
```
Unintended benefits include one fewer panicking branch in `ilog2` for LLVM to optimize away, and one fewer `assume_unchecked` as `NonZero` already does that.
The fast path for `self == 1` is dropped, but the current implementation is very slow anyways compared to hardware. Performance improvements can always come later.
(I didn't add the function to `NonZero<iN>`, since _every_ existing `NonZero` method is non-panicking, and it might be nice to leave it that way.)
355cdd3bc1