Let xt::diagonal() return results in a column major fashion

[DavisVaughan]

I think xt::diagonal() (and the rest of the builders, like xt::eye()), need a layout argument somehow.

Consider this example:

// [[Rcpp::depends(xtensor)]]
// [[Rcpp::plugins(cpp14)]]

#include <xtensor-r/rarray.hpp>
#include <xtensor/xbuilder.hpp>

// [[Rcpp::export]]
xt::rarray<int> diagonal_test(const xt::rarray<int>& x) {
  return xt::diagonal(x);
}

Rcpp::sourceCpp("~/Desktop/test.cpp")

x <- matrix(1:9, 3)
x
#>      [,1] [,2] [,3]
#> [1,]    1    4    7
#> [2,]    2    5    8
#> [3,]    3    6    9

# all good
diagonal_test(x)
#> [1] 1 5 9

y <- array(1:12, c(2, 2, 3))
y
#> , , 1
#> 
#>      [,1] [,2]
#> [1,]    1    3
#> [2,]    2    4
#> 
#> , , 2
#> 
#>      [,1] [,2]
#> [1,]    5    7
#> [2,]    6    8
#> 
#> , , 3
#> 
#>      [,1] [,2]
#> [1,]    9   11
#> [2,]   10   12

# Wrong!
diagonal_test(y)
#>      [,1] [,2]
#> [1,]    1    4
#> [2,]    5    8
#> [3,]    9   12

# I expect
matrix(c(1, 4, 5, 8, 9, 12), nrow = 2)
#>      [,1] [,2] [,3]
#> [1,]    1    5    9
#> [2,]    4    8   12

Hopefully you can see here that with the 3D array, two things are wrong:

• The shape of the container is wrong because the new dimension is added to the RHS, rather than the LHS
• Results are filled in in a row major fashion, when I wanted them to be filled in a column major fashion

I think that xtensor-stack/xtensor-r#90 might be a similar issue.