make ReverseDiff Optional

[jw3126]

Thanks a lot for creating SNOW! ReverseDiff.jl is a heavy package, but personally I almost never actually use it.
How about making it optional?

Before PR

julia> @time using SNOW
  9.064925 seconds (30.91 M allocations: 2.149 GiB, 3.74% gc time, 0.3
 which was recompilation)

After PR

julia> @time using SNOW
  3.619427 seconds (7.36 M allocations: 549.917 MiB, 8.71% gc time, 0.96% compilation time: 3% o
f which was recompilation)

[jw3126]

@andrewning are you in principle interested in making ReverseDiff.jl optional?

[andrewning]

Yeah, that's fine. Not a high priority at the moment as the startup time is a one-time cost, but when I have other updates to bundle in I'll get around to tagging a new release.

[jw3126]

Ok great!

Not a high priority at the moment as the startup time is a one-time cost

Usually yes, but in some cases, you need to restart often. E.g. when redefining structs or debugging code that hangs or crashes etc,

[jw3126]

@andrewning are you ok with merging this, even if you don't want to tag a release right away?