rust: str: add fmt! macro

Add the `fmt!` macro, which is a convenience alias for the Rust
`core::format_args!` macro.

For instance, it may be used to create a `CString`:

    CString::try_from_fmt(fmt!("{}{}", "abc", 42))?

Signed-off-by: Wedson Almeida Filho <[email protected]>
Reviewed-by: Gary Guo <[email protected]>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <[email protected]>

rust/kernel/str.rs

@@ -583,3 +583,9 @@ impl Deref for CString {
         unsafe { CStr::from_bytes_with_nul_unchecked(self.buf.as_slice()) }
     }
 }
+
+/// A convenience alias for [`core::format_args`].
+#[macro_export]
+macro_rules! fmt {
+    ($($f:tt)*) => ( core::format_args!($($f)*) )
+}