More abstract tuple types like `Tuple` or `NTuple` (without parameters) and vararg tuples remain without any iteration methods defined.

I only discovered one minor confusion stemming from a definition for `eltype` for NTuples of all the same type.  Since eltype works in the type domain, there is no way to tell whether `eltype{N,T}(::Type{NTuple{N,T}})` should be `T` or `Type{T}`. I have removed these definitions.