RFC: Mapslices for nonscalar arguments

Because of #5140 I took a detailed look at mapslices and how it arranges the results of ``f(s)``, where ``s`` denotes the slices along dimensions ``slicedims`` of array ``A``, into a new array ``R = [f(s)]``.

I should just explain it here for anyone interested.
As stated in the documentation, the ``f(s)`` are concatenated along the sliced dimensions.
The new array inherits the number of dimensions of ``A``, and the length of the dimensions not sliced over stays the same. The length of the ``n`` first dimensions in ``slicedims`` get modified to accomodate the ``r``'s.
In fact, for the result ``R`` has the shape ``sizeR`` with
```
sizeR = size(A)
sizeR[slicedims] = [size(r), fill up with ones]
```

For example if ``f`` is scalar
```
julia> mapslices(sum, rand(4,5), [1]) 
1x5 Array{Float64,2}:
 1.80329  1.98077  2.89279  1.69202  2.69841
julia> mapslices(sum, rand(4,5), [2]) 
4x1 Array{Float64,2}:
 1.84754
 2.56395
 2.14556
 2.23409
```
and the following nonscalar ``f`` centers the columns versus the rows.
```
 A = mapslices(x-> x-mean(x), rand(4,5), [2]) 
4x5 Array{Float64,2}:
  0.230468     0.0108605  -0.141878  -0.219276    0.119825
 -0.106796    -0.318773   -0.147437   0.290916    0.28209 
  0.350075    -0.261308   -0.166023  -0.0719417   0.149197
 -0.00311726  -0.352739    0.374071   0.10547    -0.123686
julia> round(sum(A,2), 8)
4x1 Array{Float64,2}:
 -0.0
 -0.0
  0.0
  0.0
```
The implementation doesn't handle A which is 3-dimensional,
because in line https://github.com/JuliaLang/julia/blob/master/base/abstractarray.jl#L1628
the left and right side differ by dimensions of length 1
```
julia> A = mapslices(x-> x-mean(x), rand(4,3,2), [2,3]) 
ERROR: argument dimensions must match
 in setindex! at array.jl:606
 in mapslices at abstractarray.jl:1631
```

A possible fix is to assign elementwise if ndims(r) > 1.