Currently, parse.y actions are hard to read and write because the code
has double meaning (for core parser and for ripper).  I think that, if
it is easy to write ripper's code shortly and simply, the double meaning
trick is not needed.

For the sake, this change adds a simple DSL for ripper's code.  For
example, in parse.y, we can write:

    /*% ripper: stmts_add(stmts_new, void_stmt) %*/

instead of:

    $$ = dispatch2(stmts_add, dispatch0(stmts_new),
                   dispatch0(void_stmt));

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61952 b2dd03c8-39d4-4d8f-98ff-823fe69b080e