Give Flatten-iterator HasLength() in some cases

[mschauer]

It is worthwhile to give some flattened iterators with elements of known type the trait HasLength.
This PR does this for iterators with elements of type tuple.

The length can be easily computed from the length of the iterator and the nfields of the tuple-type. This makes it for example possible to use a flattened zip as input of IteratorND and a naive vcat for iterators becomes simply

function itervcat(a...)
    z = zip(a...)
    Base.IteratorND(Base.flatten(z), (length(a), size(z)...))
end

This PR only gives the HasLength trait to Base.Flatten. The function itervcat just illustrates the usefulness.

The check for isleaftype gets optimized away:

@code_typed Base.iteratorsize(Base.flatten(zip(1:3)))
1-element Array{Any,1}:
 LambdaInfo for iteratorsize
:(begin  # generator.jl, line 34:
        # meta: location iterator.jl iteratorsize # iterator.jl, line 488:
        $(QuoteNode(true))
        # meta: pop location
        return $(Expr(:new, :(Base.HasLength)))
    end::Base.HasLength)

[mschauer]

Personal assessment: this change is helpful, but the scope is limited as there are few objects beyond tuples which have static length (or size) attribute, and there are little means in julia to talk about length and size of a type. On the other hand fixed size arrays and a hypothetical buffer type can also covered in a similar way in the future.

[tkelman]

should this actually throw the ArgumentError instead of returning it? and test that branch?

[mschauer]

Replaced by #22691