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Separate RDoc::TokenStream#add_tokens and #add_token

The old version of add_tokens accepts an array of tokens, and
multiple arguments of tokens by using Array#flatten.
And add_token was an alias to add_tokens.

I think it is unnecessarily flexible; in fact, all callsites of
add_tokens (except test) passes only an array of tokens.
And the code created a lot of temporal arrays.

This change makes add_tokens accept only one array of tokens,
and does add_token accept one token. It is a bit faster (about
1 second in Ruby's make rdoc), and it ls also cleaner in my point of
view.