Making getproperty, getindex and iterate more type stable for Tangents

[ToucheSir]

Constprop is rarely able to pierce the function barrier and get through to the underlying type. This would especially help when the backing is a tuple or NamedTuple.

I think the main question is how to do this. @inline as seen in https://github.com/JuliaLang/julia/blob/master/base/tuple.jl is one option. Base.@constprop :aggressive is another that Diffractor currently employs.

[oxinabox]

Sounds good, if someone is interested in doing the experiments and working it out i would be happy to merge.
Not sure when I will get around to looking at it myself.