Don't slice in lexer

Summary: Theoretically this allocates way less. In practice it seems to be exactly the same speed.

Test Plan: make test

Reviewers: emily

Reviewed By: emily

Differential Revision: https://phabricator.khanacademy.org/D16621

package.json

@@ -25,5 +25,8 @@
   "bin": "cli.js",
   "scripts": {
     "test": "make lint test"
+  },
+  "dependencies": {
+    "match-at": "^0.1.0"
   }
 }