Merge #1192

1192: Improve `restructure` performance r=dhairyagandhi96 a=MikeInnes

A small change, but it significantly improves the performance on the following test case:

```julia
julia> VERSION
v"1.5.0-DEV.876"

julia> using Flux, DiffEqFlux, BenchmarkTools

julia> using Flux: mse

julia> fastdense = FastDense(784, 32, tanh);

julia> p = initial_params(fastdense);

julia> dense = Dense(784, 32, tanh);

julia> p,re = Flux.destructure(dense);

julia> x = rand(Float32, 784, 10);

julia> y = rand(Float32, 32, 10);

julia> @btime gradient((x,p) -> mse(fastdense(x, p), y), x, p);
  505.530 μs (87 allocations: 240.73 KiB)

julia> @btime gradient((x,p) -> mse(re(p)(x), y), x, p);
  107.796 μs (139 allocations: 340.94 KiB)
```

Co-authored-by: Mike J Innes <mike.j.innes@gmail.com>

src/utils.jl

@@ -246,6 +246,10 @@ function _restructure(m, xs)
   end
 end
 
+@adjoint function _restructure(m, xs)
+  _restructure(m, xs), dm -> (nothing,destructure(dm)[1])
+end
+
 """
     destructure(m)