Special case `x^1`

[dpsanders]

x^0 is special-cased here (see #331)

Could x^1 also be special-cased a couple of lines later?
Currently it instead calculates 1 * x^0. When x == 0 that relies on 0^0 being defined correctly.

(This led to the error in #411.)

[tkf]

I think you need to special case it dynamically here

ForwardDiff.jl/src/dual.jl

Lines 452 to 453 in c374b69

	if y == zero(y)
	new_partials = zero(partials(x))

by adding elseif y === one(y) branch and also here

ForwardDiff.jl/src/dual.jl

Lines 472 to 473 in c374b69

	for y in 1:3
	@eval @inline function Base.literal_pow(::typeof(^), x::Dual{T}, ::Val{$y}) where {T}

for the literal case (x^1).