Deprecate `@_` in favor of `...`

[barefootcoder]

I've just closed out #26, which I think is right and proper. However, it occurs to me that ... is pretty much superior to @_ in every way: it performs equivalently, is recognizable as an actual operator (both in Perl 5, as of 5.12, and Perl 6), and is easier to type even though it's technically one character longer. Can we/should we go ahead and deprecate the @_ signature?

[schwern]

I'm cool with that.

If we're adding bare sigils for 5.20 compatibility, it would seem that @ takes place of both @_ and .... So we'll have THREE ways of saying "I don't care".