Support MutableTangent automatically

[oxinabox]

This is a follow up to JuliaDiff/ChainRulesCore.jl#105 (comment)

Right now, using JuliaDiff/ChainRulesCore.jl#626 if you want to test something that MutableTangent that does work e.g.:
test_frule(setfield!, MDemo(3.5)⊢MutableTangent{MDemo}(x=2.0), :x, 5.0)

But if you just do:
test_frule(setfield!, MDemo(3.5), :x, 5.0)
you get an error because the default type we use for structural tangents is the immutable Tangent

We could use the StructuralTangent constructor to automatically chose MutableTangent for mutable primals.
we would want to do that forwardward mode, at least. Not sure if we should also do that in reverse