Implemented one-char cache lookup in generated code.

src/string.js

Index: src/string.js
diff --git a/src/string.js b/src/string.js
index 49f403de7da8140bb8aeebd9d87c00c329850a0d..531c6a9b949bbb4c897c761926fcd9164cbbb024 100644
--- a/src/string.js
+++ b/src/string.js
@@ -69,7 +69,7 @@ function StringCharAt(pos) {
     if (index >= subject.length || index < 0) return "";
     char_code = %StringCharCodeAt(subject, index);
   }
-  return %CharFromCode(char_code);
+  return %_CharFromCode(char_code);
 }
 
 
@@ -184,7 +184,7 @@ function SubString(string, start, end) {
     if (!%_IsSmi(char_code)) {
       char_code = %StringCharCodeAt(string, start);
     }
-    return %CharFromCode(char_code);
+    return %_CharFromCode(char_code);
   }
   return %_SubString(string, start, end);
 }
@@ -726,7 +726,7 @@ function StringTrimRight() {
 // ECMA-262, section 15.5.3.2
 function StringFromCharCode(code) {
   var n = %_ArgumentsLength();
-  if (n == 1) return %CharFromCode(ToNumber(code) & 0xffff)
+  if (n == 1) return %_CharFromCode(ToNumber(code) & 0xffff)
 
   // NOTE: This is not super-efficient, but it is necessary because we
   // want to avoid converting to numbers from within the virtual