RFC: Add native Array slices/views with shared data

Add method `arrayslice(A, pos, dim)` which returns a slice of type `Array` and is memory safe. The slices use contiguous parts of memory from A and can be any dimensional. The advantage for this special case compared to ArrayViews/SubArrays is performance and compatibility with any code for Arrays.

The C function `jl_slice_owned_array` is actually a general case of `jl_reshape_array`, so there is a question whether the code duplication is ok, or whether to make the reshape function call the slice function.

Memory check (same as with `reshape`):
```julia
n=int(1e8)
x=rand(n);
gc() # nothing happens
y=arrayslice(x,101,20)
gc() # nothing happens
x=1
gc() # nothing happens
y=1
gc() # memory usage reduces
```
Code example:
```julia
julia> x=rand(3,5)
3x5 Array{Float64,2}:
 0.839011   0.954     0.185297  0.236349  0.0152856
 0.0864201  0.561014  0.215385  0.219099  0.948724 
 0.708425   0.489022  0.20037   0.907779  0.399409 

julia> y=arrayslice(x,7,(3,3))
3x3 Array{Float64,2}:
 0.185297  0.236349  0.0152856
 0.215385  0.219099  0.948724 
 0.20037   0.907779  0.399409 

julia> I=eye(3);

julia> copy!(y,I);

julia> x
3x5 Array{Float64,2}:
 0.839011   0.954     1.0  0.0  0.0
 0.0864201  0.561014  0.0  1.0  0.0
 0.708425   0.489022  0.0  0.0  1.0

julia> y=arrayslice(x,1,3)
3-element Array{Float64,1}:
 0.839011
 0.0864201
 0.708425

julia> scale!(100,y);

julia> x
3x5 Array{Float64,2}:
 83.9011   0.954     1.0  0.0  0.0
  8.64201  0.561014  0.0  1.0  0.0
 70.8425   0.489022  0.0  0.0  1.0
```