Syntax for tuples

[alexcrichton]

Right now the syntax for tuples is tuple<A, B, C>, but the only reason for this was to disambiguate the multi-value returns we originally had which had the shape -> (A, B, C). It's probably worth revisiting this decision and considering (A, B, C) for tuples now instead of the tuple<...> syntax.

[alexcrichton]

I believe this will be handled by #299, so closing in favor of that.