Formatter::with_options: Use different lifetimes

Formatter::with_options takes self as a mutable reference (`&'a mut
Formatter<'b>`). `'a` and `'b` need to be different lifetimes. Just taking `&'a
mut Formatter<'a>` and trusting in Rust being able to implicitely convert from
`&'a mut Formatter<'b>` if necessary (after all, `'a` must be smaller than `'b`
anyway) fails because `'b` is behind a *mutable* reference. For background on
on this behavior, see https://doc.rust-lang.org/nomicon/subtyping.html#variance.

core/src/fmt/mod.rs

@@ -530,7 +530,7 @@ impl<'a> Formatter<'a> {
 
     /// Creates a new formatter based on this one with given [`FormattingOptions`].
     #[unstable(feature = "formatting_options", issue = "118117")]
-    pub fn with_options(&'a mut self, options: FormattingOptions) -> Self {
+    pub fn with_options<'b>(&'b mut self, options: FormattingOptions) -> Formatter<'b> {
         Formatter { options, buf: self.buf }
     }
 }