Improve keyed loads on strings by using a new stub

src/runtime.js

Index: src/runtime.js
===================================================================
--- src/runtime.js	(revision 3546)
+++ src/runtime.js	(working copy)
@@ -474,6 +474,17 @@
 }
 
 
+// Specialized version of String.charAt. It assumes string as
+// the receiver type and that the index is a number.
+function STRING_CHAR_AT(pos) {
+  var char_code = %_FastCharCodeAt(this, pos);
+  if (!%_IsSmi(char_code)) {
+    return %StringCharAt(this, pos);
+  }
+  return %CharFromCode(char_code);
+}
+
+
 /* -------------------------------------
    - - -   C o n v e r s i o n s   - - -
    -------------------------------------