append! doesn't follow AbstractArray interface, relies on Tables instead

Found a bug in the `append!` function of StructArrays:
```julia
julia> using AxisKeys, StructArrays

# OK:
julia> append!(StructArray(a=[1,2]), [(a=3,),(a=4,)])
4-element StructArray(::Vector{Int64}) with eltype NamedTuple{(:a,), Tuple{Int64}}:
...

# error:
julia> append!(StructArray(a=[1,2]), KeyedArray([(a=3,),(a=4,)], 1:2))
ERROR: type NamedTuple has no field a
```