Avoid unnecessary internings.

Most involving `Symbol::intern` on string literals.

src/libsyntax/std_inject.rs

@@ -20,9 +20,7 @@ fn ignored_span(sp: Span, edition: Edition) -> Span {
         call_site: DUMMY_SP,
         def_site: None,
         format: MacroAttribute(Symbol::intern("std_inject")),
-        allow_internal_unstable: Some(vec![
-            Symbol::intern("prelude_import"),
-        ].into()),
+        allow_internal_unstable: Some(vec![sym::prelude_import].into()),
         allow_internal_unsafe: false,
         local_inner_macros: false,
         edition,
@@ -98,7 +96,7 @@ pub fn maybe_inject_crates_ref(
     krate.module.items.insert(0, P(ast::Item {
         attrs: vec![ast::Attribute {
             style: ast::AttrStyle::Outer,
-            path: ast::Path::from_ident(ast::Ident::new(Symbol::intern("prelude_import"), span)),
+            path: ast::Path::from_ident(ast::Ident::new(sym::prelude_import, span)),
             tokens: TokenStream::empty(),
             id: attr::mk_attr_id(),
             is_sugared_doc: false,