Storage should be implemented as an unsafe trait

[vbarrielle]

See rust-lang/rust#29701
The implementation in bluss/rust-ndarray looks like a good example of how to do it.

[bluss]

Maybe it doesn't matter if all constructors are locked down?

[vbarrielle]

Maybe indeed. Having a dedicated trait can be beneficial in some cases though.
For instance, a sparse vector can be seen as a sparse matrix, but currently
this is done by creating a sparse matrix with Vec as a backend for the indptr
storage. Having a trait would enable having a [usize; 2] for the indptr.

[vbarrielle]

Since the only valid reason was the matrix view around a vector, #111 fixes this issue.