Update `ForwardDiff` dependency

[eschnett]

ForwardDiff version 1 is available.

[eschnett]

Note that there is one breaking change; see https://github.com/JuliaDiff/ForwardDiff.jl/releases/tag/v1.0.0.

[moble]

This was #76, which was closed already. I forgot to make a new release, but I'm doing it now! :)