rust: str: make RawFormatter::bytes_written public.

rnull is going to make use of `kernel::str::RawFormatter::bytes_written`,
so make the visibility public.

Signed-off-by: Andreas Hindborg <[email protected]>

Author: Andreas Hindborg

rust/kernel/str.rs

@@ -761,7 +761,7 @@ impl RawFormatter {
     }
 
     /// Returns the number of bytes written to the formatter.
-    pub(crate) fn bytes_written(&self) -> usize {
+    pub fn bytes_written(&self) -> usize {
         self.pos - self.beg
     }
 }