Improve performance of Symbol::print_value_on()

During investigation of performance of class unloading I found that logging the unloading event (UnloadingEventLog::log()) takes 20-40% of total class unloading time.

The problem seems to be Symbol::print_value_on().

The code is:

void Symbol::print_value_on(outputStream* st) const {
  st->print("'");
  for (int i = 0; i < utf8_length(); i++) {
    st->print("%c", char_at(i));
  }
  st->print("'");
}

I.e. ultimately uses snprintf for every character of the symbol. The code seems to print the raw utf-8 string (ju)byte by (ju)byte.

A faster way seems to be just use outputStream::write() since apparently the code wants to print the raw utf8 bytes anyway:

  st->write("'", 1);
  st->write((const char*)base(), utf8_length());
  st->write("'", 1);

Initial tests showed that this is ~5-10x faster, reducing the overhead of event logging during class unloading significantly.