remove several uses of unsafe (#8600)

This PR removes several uses of `unsafe`. I generally limited myself to
low hanging fruit that I could see. There are still a few remaining uses
of `unsafe` that looked a bit more difficult to remove (if possible at
all). But this gets rid of a good chunk of them.

I put each `unsafe` removal into its own commit with a justification for
why I did it. So I would encourage reviewing this PR commit-by-commit.
That way, we can legislate them independently. It's no problem to drop a
commit if we feel the `unsafe` should stay in that case.

crates/ruff_python_parser/src/string.rs

@@ -120,10 +120,8 @@ impl<'a> StringParser<'a> {
             len += 1;
         }
 
-        // SAFETY: radix_bytes is always going to be in the ASCII range.
-        #[allow(unsafe_code)]
-        let radix_str = unsafe { std::str::from_utf8_unchecked(&radix_bytes[..len]) };
-
+        // OK because radix_bytes is always going to be in the ASCII range.
+        let radix_str = std::str::from_utf8(&radix_bytes[..len]).expect("ASCII bytes");
         let value = u32::from_str_radix(radix_str, 8).unwrap();
         char::from_u32(value).unwrap()
     }