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_literal/src/escape.rs

@@ -415,12 +415,10 @@ impl<'a> Escape for AsciiEscape<'a> {
     fn layout(&self) -> &EscapeLayout {
         &self.layout
     }
-    #[allow(unsafe_code)]
     fn write_source(&self, formatter: &mut impl std::fmt::Write) -> std::fmt::Result {
-        formatter.write_str(unsafe {
-            // SAFETY: this function must be called only when source is printable ascii characters
-            std::str::from_utf8_unchecked(self.source)
-        })
+        // OK because function must be called only when source is printable ascii characters.
+        let string = std::str::from_utf8(self.source).expect("ASCII bytes");
+        formatter.write_str(string)
     }
 
     #[cold]