Narrow AbstractArray to AbstractArray{<:Number} in jacobian

[mcabbott]

Aims to address #1581:

julia> using Zygote

julia> jacobian(vv -> prod(vv[1]), [[1,2,3.], [4,5,6.]])
(Any[0 0],)  # before
(nothing,)  # after

julia> gradient(vv -> prod(vv[1]), [[1,2,3.], [4,5,6.]])  # unchanged
(Union{Nothing, Vector{Float64}}[[6.0, 3.0, 2.0], nothing],)

Also change this failure case, of a function which returns an array of arrays:

julia> jacobian(x -> [x, x./2], [1,2,3.])
ERROR: MethodError: no method matching zero(::Type{Vector{Float64}})

julia> using StaticArrays

julia> jacobian(x -> [x, x./2], SA[1,2,3.])
ERROR: DimensionMismatch: No precise constructor for SVector{3, Float64} found. Length of input was 1.
Stacktrace:
...
  [9] _eyelike(y::Vector{SVector{3, Float64}})
    @ Zygote ~/.julia/packages/Zygote/55SqB/src/lib/grad.jl:204
 [10] withjacobian(f::Function, args::SVector{3, Float64})
    @ Zygote ~/.julia/packages/Zygote/55SqB/src/lib/grad.jl:188

... to have the more obvious error:

julia> jacobian(x -> [x, x./2], [1,2,3.])
ERROR: ArgumentError: jacobian expected a function which returns an array, or a scalar, got Vector{Vector{Float64}}