string_bytes: use extern for length and write utf8

If the string is external then the length can be quickly retrieved. This
is especially faster for large strings that are being treated as UTF8.
Also, if the string is external then there's no need for a full
String::WriteUtf8 operation. A simple memcpy will do.

src/string_bytes.cc

@@ -273,7 +273,7 @@ size_t StringBytes::Write(char* buf,
   size_t len = 0;
   bool is_extern = GetExternalParts(val, &data, &len);
 
-  Local<String> str = val->ToString();
+  Local<String> str = val.As<String>();
   len = len < buflen ? len : buflen;
 
   int flags = String::NO_NULL_TERMINATION |
@@ -295,7 +295,10 @@ size_t StringBytes::Write(char* buf,
       break;
 
     case UTF8:
-      len = str->WriteUtf8(buf, buflen, chars_written, flags);
+      if (is_extern)
+        memcpy(buf, data, len);
+      else
+        len = str->WriteUtf8(buf, buflen, chars_written, flags);
       break;
 
     case UCS2:
@@ -403,9 +406,12 @@ size_t StringBytes::Size(Handle<Value> val, enum encoding encoding) {
   size_t data_size = 0;
   bool is_buffer = Buffer::HasInstance(val);
 
-  if (is_buffer && (encoding == BUFFER || encoding == BINARY)) {
+  if (is_buffer && (encoding == BUFFER || encoding == BINARY))
     return Buffer::Length(val);
-  }
+
+  const char* data;
+  if (GetExternalParts(val, &data, &data_size))
+    return data_size;
 
   Local<String> str = val->ToString();