Fix empty tuple piracy

[kleinschmidt]

Fixes #213

The type aliases for tuples of terms used NTuple which included empty
tuples. This changes those aliases to use Tuple{T, Vararg{T}} instead, which
enforces that there's at least one T (AbstractTerm etc.) in the tuple.

[NHDaly]

Awesome, thanks! 🎉

Do note that this means that your other pirated functions, like () + (), will no longer work. You might find that users were relying on this without realizing it if they had some tuples constructed dynamically and they end up empty?

Something to keep an eye on. 👍 Thanks!

[kleinschmidt]

I certainly look forward to those bug reports, if only to find out what kind of strange use cases people have come up with :)