more clippy::style fixes:

get_first
single_char_add_str
unnecessary_mut_passed
manual_map
manual_is_ascii_check

compiler/rustc_hir/src/hir.rs

@@ -3013,8 +3013,7 @@ pub struct FieldDef<'hir> {
 impl FieldDef<'_> {
     // Still necessary in couple of places
     pub fn is_positional(&self) -> bool {
-        let first = self.ident.as_str().as_bytes()[0];
-        (b'0'..=b'9').contains(&first)
+        self.ident.as_str().as_bytes()[0].is_ascii_digit()
     }
 }