Fix curl calling 3d function for 1d and 2d vectors

[KnutAM]

Noticed that

curl(x -> rotate(x, angle::Real), rand(Vec{2}))

failed since rotate(::Vec{3}, angle::Real) is not defined.

The new implementation should be faster as well, se below.

[KnutAM]

Benchmarks,

julia> foo(x::Vec) = (x[1] * x[2]) * x
foo (generic function with 1 method)

julia> x = rand(Vec{2});

julia> @btime curl($foo, $x);
  1.916 ns (0 allocations: 0 bytes) # PR 
  3.542 ns (0 allocations: 0 bytes) # master

[KnutAM]

Ready for review, @fredrikekre or @KristofferC