code: @Override
      public FastStr replaceFirst(String regex, String replacement) {
          return unsafeOf(Pattern.compile(regex).matcher(this).replaceFirst(replacement));
      }
docs: Wrapper of {@link String#replaceFirst(String, String)} but return FastStr inance
  
  @param regex the regular expression specifies the place to be replaced
  @param replacement the string to replace the found part
  @return a FastStr that has the first found part replaced