We keep __fish_is_nth_token for compatibility and edit the
implementations of __fish_is_nth_token, __fish_is_first_token and
__fish_is_token_n to use fish_is_nth_token
__fish_complete_suffix accepts a first argument containing a
brace-expansion, like
__fish_complete_suffix '.{c,cpp,py}'
We're gonna be removing the `eval` that does that shortly, so let's
remove all uses in our code.
