Defining `length` and `prod` for `TypeVar`s that happen to be tuple-of-Ints

[timholy]

I'm not sure if this is feasible, but when one is using a tuple-of-Ints as a parameter, it would be nice to be able to use length and prod work. Then one could have deleted several type parameters from the definition of fixed-size arrays in #7568. For example, the definitions might have been

abstract AbstractFixedArray{T,SZ} <: DenseArray{T,length(SZ)}

immutable FixedArrayI{T,SZ} <: AbstractFixedArray{T,SZ}
    data::NTuple{prod(SZ),T}
end

immutable FixedArrayM{T,SZ} <: AbstractFixedArray{T,SZ}
    data::Array{T,length(SZ)}
end

saving 2 and 1 parameters, respectively.

[JeffBezanson]

Although not quite the same, I think #3766 is more realistic. Running arbitrary code to derive the supertype of a type is a tall order.

[timholy]

I wondered how one would even go about defining these, and figured that might be a problem. I'm going to close this in preference to #3766.