Fast trivial `cheb2jac`

[dlfivefifty]

julia> x = randn(100); @time cheb2jac(x, -1/2,-1/2);
  0.000522 seconds (18 allocations: 1.234 KiB)

julia> x = randn(1000); @time cheb2jac(x, -1/2,-1/2);
  0.025063 seconds (18 allocations: 8.297 KiB)