Allow creating matrix iter with an owned view

[yotamofek]

Fixes #747

First of all - thanks for this crate, it's really fun to use, except for the lack of IntoIterator impls on (owned) view storages.

This allows overcoming the borrow checker issues when trying to do something like this:

Using the IntoIterator impl from this PR:

I implemented this feature by extracting a common subset of MatrixIter and MatrixIterMut into a RawIter struct which erases the lifetime it is created with, and iterates over raw pointers instead of references.
Then, the MatrixIter(Mut) structs become simple wrappers over RawIter, but with an added lifetime constraint.

It's technically possible to keep the old code (and remove most of the code added in this PR) and impl the new new_owned and new_owned_mut using a transmute from MatrixIter<'outer, ...> to a MatrixIter<'inner, ...>, but I'm not even sure that's not possibly UB.

Let me know what you think :)

[sebcrozet]

Thanks!