Symbolics.jl compatibility

Arithmetic operations between Num and Dual based on Num is not clearly defined. The basic usecase is this:

using ForwardDiff, Symbolics.jl

@variables a, x
f = x -> a*x
Differential(x)(f(x)) |> expand_derivatives # This works, and gives what I would expect from ForwardDiff.derivative
ForwardDiff.derivative(f, x) # This returns an error

The error given by the last line is this

ERROR: MethodError: *(::Num, ::ForwardDiff.Dual{ForwardDiff.Tag{var"#5#6", Num}, Num, 1}) is ambiguous.

Candidates:
  *(x::Real, y::ForwardDiff.Dual{Ty}) where Ty
    @ ForwardDiff ~/.julia/packages/ForwardDiff/PcZ48/src/dual.jl:145
  *(a::Num, b::Real)
    @ Symbolics ~/.julia/packages/SymbolicUtils/ssQsQ/src/methods.jl:73
  *(a::Num, b::Number)
    @ Symbolics ~/.julia/packages/SymbolicUtils/ssQsQ/src/methods.jl:75

Possible fix, define
  *(::Num, ::ForwardDiff.Dual{Ty}) where Ty

Stacktrace:
 [1] (::var"#5#6")(x::ForwardDiff.Dual{ForwardDiff.Tag{var"#5#6", Num}, Num, 1})
   @ Main ./REPL[21]:1
 [2] derivative(f::var"#5#6", x::Num)
   @ ForwardDiff ~/.julia/packages/ForwardDiff/PcZ48/src/derivative.jl:14
 [3] top-level scope
   @ REPL[22]:1

This seems to be the case for (at least) all basic arithmetic operations. Not sure if this should be fixed here or in Symbolics.jl.

Reproduce the example using ForwardDiff.derivative, Symbolics.jl, Num, and ForwardDiff.Dual, then inspect the conflicting arithmetic methods shown in the error from ForwardDiff and SymbolicUtils. Determine whether the compatibility change belongs in ForwardDiff.jl or Symbolics.jl, and verify that the basic arithmetic operations no longer produce an ambiguous-method error.