Resolve manual_strip clippy lint

    warning: stripping a prefix manually
      --> src/ident_fragment.rs:53:31
       |
    53 |             fmt::Display::fmt(&id[2..], f)
       |                               ^^^^^^^^
       |
    note: the prefix was tested here
      --> src/ident_fragment.rs:52:9
       |
    52 |         if id.starts_with("r#") {
       |         ^^^^^^^^^^^^^^^^^^^^^^^^
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_strip
       = note: `-W clippy::manual-strip` implied by `-W clippy::all`
    help: try using the `strip_prefix` method
       |
    52 ~         if let Some(<stripped>) = id.strip_prefix("r#") {
    53 ~             fmt::Display::fmt(<stripped>, f)
       |

    warning: stripping a prefix manually
       --> src/runtime.rs:447:24
        |
    447 |         Ident::new_raw(&id[2..], span)
        |                        ^^^^^^^^
        |
    note: the prefix was tested here
       --> src/runtime.rs:446:5
        |
    446 |     if id.starts_with("r#") {
        |     ^^^^^^^^^^^^^^^^^^^^^^^^
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_strip
    help: try using the `strip_prefix` method
        |
    446 ~     if let Some(<stripped>) = id.strip_prefix("r#") {
    447 ~         Ident::new_raw(<stripped>, span)
        |

src/ident_fragment.rs

@@ -49,8 +49,8 @@ impl IdentFragment for Ident {
 
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         let id = self.to_string();
-        if id.starts_with("r#") {
-            fmt::Display::fmt(&id[2..], f)
+        if let Some(id) = id.strip_prefix("r#") {
+            fmt::Display::fmt(id, f)
         } else {
             fmt::Display::fmt(&id[..], f)
         }

src/runtime.rs

@@ -443,8 +443,8 @@ pub fn mk_ident(id: &str, span: Option<Span>) -> Ident {
 }
 
 fn ident_maybe_raw(id: &str, span: Span) -> Ident {
-    if id.starts_with("r#") {
-        Ident::new_raw(&id[2..], span)
+    if let Some(id) = id.strip_prefix("r#") {
+        Ident::new_raw(id, span)
     } else {
         Ident::new(id, span)
     }