`StaticArray` of `TrackedReal`s doesn't currently work (and I think it could)

Sorry for the lack of minimal example here, I'll add it when I get the chance.

I recently tried to differentiate through operations on StaticArrays.SVectors and hit two issues, both of which were easily fixable once diagnosed (unless these solutions cause other problems I'm not aware of):

1. StaticArrays defines a catch-all +(::StaticArray, ::AbstractArray) and ReverseDiff defines a catch-all +(::AbstractArray, ::TrackedArray). So if +(::StaticArray, ::TrackedArray) is called, Julia complains that the dispatch rule is ambiguous. (Same for the other order of operands too.) The same is the case for -, and I'm guessing also for *.

2. This line
   https://github.com/JuliaDiff/ReverseDiff.jl/blob/71c5ac0cb2212f49e3b3d567b45a830614173913/src/tracked.jl#L225
   assumes that similar(t) supports setindex!. In the case of StaticArrays, this assumption breaks if isbitstype(eltype(t)) is false, and in particular if t is an StaticArray of TrackedReals. Similar issue on this line:
   https://github.com/JuliaDiff/ReverseDiff.jl/blob/71c5ac0cb2212f49e3b3d567b45a830614173913/src/tracked.jl#L166

I was able to fix these locally by:

1. Explicitly defining +(::StaticArray, ::TrackedArray) and +(::TrackedArray, ::StaticArray), to disambiguate the dispatch rulea similarly for - (and probably also for *, though I didn't have the need for that). In order to not encounter the setindex! issue, I had to cast the StaticArray to an Array first.

2. 1. Redefining ReverseDiff.capture on StaticArrays to do the same thing without mutation (using map + array conversion instead of similar + map!). As an aside, if this is equally efficient, seems like we might as well make this the default definition.
   2. Redefining deriv!(::StaticArray ::AbstractArray) to apply deriv! elementwise, like it would if the argument were a Tuple.

Start at src/tracked.jl lines 166 and 225, then reproduce the reported failures with StaticArrays.SVector and StaticArray values containing TrackedReal elements. Check the relevant StaticArrays and ReverseDiff method interactions; done means supported addition and subtraction dispatches no longer conflict and capture or deriv! handles these arrays without assuming mutable similar storage.