RFC: Tuple indexing

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I agree that `a.1.3` should be legal and equivalent to `(a.1).3`, but isn't there a way to do this without introducing the awkwardness of float literals in the first place? I.e. just say that valid tokens following the `.` operator are either an identifier (named struct field, method) or a decimal number? And `a.1.3` is just the same thing twice.