#481 `iszero` workaround

Like a couple other people here, I am experiencing some issues with #481. My issue is in BesselK.jl, where I have some very specific methods that branch on an iszero(x) that were broken (for example).

I understand the logic of the change and clearly an argument can be made for it, but it is disappointing that the only solution I can come up with now is to bring ForwardDiff into the dependency tree and define ugly methods

  _iszero(x) = iszero(x)
  _iszero(x::ForwardDiff.Dual) = _iszero(ForwardDiff.value(x))

I am sure that it's too late to argue for just reverting #481, but it would be nice if there were some way to check that ForwardDiff.value(x) is zero without having to bring ForwardDiff into the tree. Is there some other generic comparison method in Base or something that ForwardDiff can add methods to that checks for zero values? Or is there some getter function that one can use to get ForwardDiff.value(x)? I'm very happy to write a different check than iszero---but I think it would be nice for the check to be possible without bringing the internal getters of ForwardDiff.jl into scope.

Excuse me if I'm using any incorrect terminology and thanks in advance for any thoughts you can share on this.

Start with the reported call in BesselK.jl, src/besk_temme.jl around line 43, and inspect how ForwardDiff.Dual currently interacts with iszero and ForwardDiff.value. Clarify whether a supported generic zero-value check or public accessor is intended. Done means an agreed API or documented approach that avoids requiring users to depend on ForwardDiff internals.