Change getproperty to getfield

[mcabbott]

Closes #46

Is this going to have any surprising consequences?

Note that @functor cannot go the other way, and use getproperty/ propertynames for everything, as that needs an instance not the type.

(Could add tests but they'd be kind-of contrived, I mean all existing tests check that this runs, new ones would only check against someone editing these lines back to getproperty.)

[ToucheSir]

LGTM. Slightly off topic, is there a way to add an org-wide breaking label so we can track what goes into such releases? And/or maybe we should use milestones?

[mcabbott]

Could do milestones. Flux seems messy enough that they help, perhaps this is simple enough not to?

[mcabbott]

This breaks the following use, which relies on Tangent overloading getproperty:

julia> using ChainRulesCore

julia> x = (a = 1,);

julia> gx = ProjectTo(x)((a=0.1,))
Tangent{NamedTuple{(:a,), Tuple{Int64}}}(a = 0.1,)

julia> gx.a
0.1

julia> functor(typeof(x), gx)
ERROR: type Tangent has no field a

I think that means we should revert it. It will break Optimisers + non-Zygote completely.