fix(rust) emoji supported in single quote strings

Author: joshgoebel

src/languages/rust.js

@@ -199,10 +199,19 @@ export default function(hljs) {
         illegal: null
       }),
       {
-        className: 'string',
+        scope: 'string',
         variants: [
           { begin: /b?r(#*)"(.|\n)*?"\1(?!#)/ },
-          { begin: /b?'\\?(x\w{2}|u\w{4}|U\w{8}|.)'/ }
+          {
+            begin: /b?'/,
+            end: /'/,
+            contains: [
+              {
+                scope: "char.escape",
+                match: /\\(\w|x\w{2}|u\w{4}|U\w{8})/
+              }
+            ]
+          }
         ]
       },
       {