Replace checked slice indexing by unchecked to support panic-free code

Fixes rust-lang#126425

Replace the potentially panicking `[]` indexing with `get_unchecked()`
to prevent linking with panic-related code.

Author: Eugene Shamis

core/src/fmt/num.rs

@@ -88,7 +88,9 @@ unsafe trait GenericRadix: Sized {
                 };
             }
         }
-        let buf = &buf[curr..];
+        // SAFETY: `curr` is initialized to `buf.len()` and is only decremented,
+        // so it is always in bounds.
+        let buf = unsafe { buf.get_unchecked(curr..) };
         // SAFETY: The only chars in `buf` are created by `Self::digit` which are assumed to be
         // valid UTF-8
         let buf = unsafe {