Row-by-row version of from_iterator

[RReverser]

Forgive me if this already exists, because I couldn't find it in the docs, but is there a way to construct a matrix row-by-row using an iterator? (for cases when source data structure already represents them in row-by-row form)

Currently the only way I found is creating matrix NxM using from_iterator, and then invoking transpose to get MxN, but this is rather inefficient - first, it needs to fill in matrix in incorrect order, then construct a separate matrix (transpose_mut fails because it says it works only on square matrices), and then walk over all the temporary data rearranging items in correct order.

[sebcrozet]

Hi! You are correct, there is no direct constructor from an iterator yielding data in a row-major order. The workaround you found Matrix::from_iterator(iter).transpose() is probably the easiest way of doing this right now. An alternative is to collect the iterator into a Vec and use Matrix::from_row_slice.
We should add a ::from_row_iterator constructor.

[chammika-become]

@sebcrozet
I had a go at this. First I thought this is going to be a convenience function around Matrix::from_iterator(iter).transpose() as hinted above. But it end up being little bit more... so I am not sure if it's the right approach.
My changes dev...chammika-become:from_row_iterator required following.

• add allocate_from_row_iterator to Allocator trait
• Implement the this for DefaultAllocator variations (static x static, static x dynamic and dynamic x static)
• Use it to implement ::from_row_iterator

Let me know if there is a simpler approach to this. Otherwise I am glad to make a PR from this.

[Trolloldem]

@sebcrozet
I noticed that the changes requested in #888 are not being addressed since when they were added to the PR a couple of months ago. I had a go at implementing them. The PR walks through the steps that I've followed.

Let me know if these are all the requested changes. Otherwise I can improve from that code.