Replace StaticArrays dependency with `ismutable`

[gdalle]

Maybe we could implement the mutable and immutable result types by checking ismutable(value) and any(ismutable, derivs)?
It's not type-inferrable in the strict sense but perhaps constant propagation will get us most of the way there?

[devmotion]

AFAICT ismutable is not sufficient to determine whether an array is mutable, ie. whether setindex! is supported - it seems to be merely a trait to distinguish between (im)mutable types. For instance, according to ismutable immutable wrapper types are not mutable:

julia> A = rand(3, 1)';

julia> ismutable(A)
false

julia> A[1, 2]
0.8835110796659092

julia> A[1, 2] = 0.0
0.0

julia> A[1, 2]
0.0

You'd need something like ArrayInterface.ismutable:

julia> using ArrayInterface

julia> ArrayInterface.ismutable(A)
true

[gdalle]

That's fair, although mildly disappointing I guess ^^