add prettier printing for duals

[KristofferC]

With inspiration taking from the complex pretty printing in base:

julia> ForwardDiff.Dual(3, 1, 2)
3 + 1ɛ₁ + 2ɛ₂

julia> ForwardDiff.Dual(3, 1)
3 + 1ɛ

julia> ForwardDiff.Dual(3, -1)
3 - 1ɛ

julia> ForwardDiff.Dual(3, 1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 2)
3 + 1ɛ₁ + 2ɛ₂ + 3ɛ₃ + 4ɛ₄ + 5ɛ₅ + 6ɛ₆ + 7ɛ₇ + 8ɛ₈ + 9ɛ₉ + 1ɛ₁₀ + 2ɛ₁₁

Will add tests after obligatory bikeshedding :)

[KristofferC]

Perhaps better to implement show for the partials instead.

[jrevels]

I actually had something similar to this at one point, except it would parenthesize epsilon components for nested dual numbers. I got rid of it in favor of the existing behavior, which I now find much more legible for actual work. I really don't want to go back to drowning in epsilon soup...

I appreciate your work here, so sorry to be a downer, but it's probably going to get a 👎 from me unless there are clearer examples of where this pretty printing is more legible than the current pretty printing.

[KristofferC]

Sounds reasonable. I'm just procrastinating here anyway ;)