Add features required by rusty-machine.

[sebcrozet]

See #498.

[jswrenn]

It would be slightly more idiomatic to name these iterators Rows & RowsMut, and Cols and ColsMut.

Some examples of this pattern in the standard library include Chars and Values and ValuesMut.

[sebcrozet]

@jswrenn I believe the convention is for the iterator type to have the same name as the method returning it. In our case the methods are row_iter(), row_iter_mut() etc, hence the RowIter, RowIterMut, etc names.

I didn't call those methods rows and rows_mut in the first place because those names are already used for slicing the matrix.

[jswrenn]

I forgot about those methods! You're right about the convention. The name conflict is unfortunate.

[sebcrozet]

I'm merging this for today's release.
#499 is not fully fixed yet so we will open another PR to continue the work.