Implemented one-char cache lookup in generated code.

src/runtime.js

Index: src/runtime.js
diff --git a/src/runtime.js b/src/runtime.js
index e9d98487721a07a1abe004fbde6377b0710b5c64..d7770414a3b2376c408abdc271025de80629885c 100644
--- a/src/runtime.js
+++ b/src/runtime.js
@@ -478,7 +478,7 @@ function STRING_CHAR_AT(pos) {
   if (!%_IsSmi(char_code)) {
     return %StringCharAt(this, pos);
   }
-  return %CharFromCode(char_code);
+  return %_CharFromCode(char_code);
 }