Change `Token::interpolated_to_tokenstream()`.

Change `Token::interpolated_to_tokenstream()`.

It is currently a method of `Token`, but it only is valid to call if
`self` is a `Token::Interpolated`. This commit eliminates the
possibility of misuse by changing it to an associated function that
takes a `Nonterminal`, which also simplifies the call sites.

This requires splitting out a new function, `nonterminal_to_string`.