Support automatic differentiation with Zygote

Supporting reverse-mode autodiff with Zygote requires two things:

• custom pullbacks (reverse-mode differentiation rules, AKA adjoints) for functions that internally mutate arrays (Zygote does not support array mutation)
• custom pullbacks for constructors

I think the latter is the main thing missing for Zygote support. Supporting it would require adding custom rrules for constructors using ChainRulesCore, which has essentially no dependencies.

Here's an example that fails:

julia> using Rotations, Zygote

julia> foo(ω, v) = (RotationVec(ω...) * v)[1];

julia> ω, v = randn(3), randn(3)
([-0.5124874613220701, 0.27274002772526423, 1.0593705312514463], [-0.2329525748114141, -1.1183670007323072, -0.4878065893106537])

julia> foo(ω, v)
0.8904209135865043

julia> Zygote.gradient(foo, ω, v) # expected from finite differences: ([-0.18142454901446126, -0.16476417425397563, 0.8182962760715483], [0.47098619925782836, -0.8816660765248943, -0.028929735807022527])
ERROR: Need an adjoint for constructor RotationVec{Float64}. Gradient is of type Array{Float64,2}

In this case mathematically the missing pullback is for the exponential map exp: so(3) → SO(3).

Start by reproducing the Julia/Zygote example in the issue and inspect the RotationVec constructor, along with ChainRulesCore's rrule mechanism. Done means constructor pullbacks are available for the relevant rotation types, including the exponential-map case, and the example can compute the expected gradients without an adjoint error.