Fix ReverseDiff's `value_and_gradient!`

As diagnosed in #386, the function `value_and_gradient!(f, grad, ::AutoReverseDiff, x)` returns wrong primal outputs `y = f(x)` due to https://github.com/JuliaDiff/ReverseDiff.jl/issues/251. #386 fixes it, but at the cost of one extra function call. The long-term fix is something like https://github.com/JuliaDiff/ReverseDiff.jl/pull/261