`:nodoc:` seems not working for inner classes.

The allocated parser string is never freed, which causes a memory leak.

The following code leaks memory:

    Ripper.sexp_raw(DATA.read)

    __END__
    <<~EOF
      a
        #{1}
      a
    EOF

This change is reduce Ruby C API dependency for Universal Parser.
Reuse dedent_string functions in rb_ruby_ripper_dedent_string functions and remove dependencies on rb_str_modify and rb_str_set_len from the parser.

Fallback to the last token element in that case, for the backward
compatibilities.

Now it is used only for wheter `opt_paren_args` is `none`.  Introduce
a new special node to distinguish an empty parentheses from it .