Lift `ndarray` compatibility into `Buffer<T>`

[metasim]

• [X] I agree to follow the project's code of conduct.

• [X] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

• Added ability to convert between Buffer<T> and ndarray::Array2<T>.

• Implemented IntoIterator, Index and IndexMut for Buffer<T>.

• Breaking: Buffer<T>::size is now private and accessed via `Buffer::shape().

• Breaking: Buffer<T>::data is now private and accessed via `Buffer::data().

• Breaking: Removed Rasterband::read_as_array, changed signature of Rasterband::read_block to return a Buffer<T>.

• Breaking: Rasterband::write and Rasterband::write_block now require a &mut Buffer<T> to handle possible case of drivers temporarily mutating input buffer.

cc: @mwielocha

[lnicola]

I'll have to think a bit more about this one, but I'm not sure I'm a fan. We should probably enable conversion from Buffer to Array2 and back instead of having two separate read and write functions.

But more importantly, we should have a way to pass in a buffer instead of allocating a new one each time.

[metasim]

@lnicola Let me restate the use case just to make sure it's clear what I'm trying to do here.

I have an application where I do not want to include/use the ndarray library, and I want to be able to read an image block by block. Before this PR I could not do that. Currently, block reading is only available if you are an ndarray user.

The renaming of the existing one is clearly not required, so I can figure out some other naming if that's objectionable.

[metasim]

I just re-read your comment. If what you're arguing is that we should normalize the I/O around the Buffer<T> type (or, perhaps even better, Vec<T>!), then I'm even more of a fan of that, but wondered how controversial that would be.

Instantiating ArrayX<T> from a Vec<T> is so easy, it does seem silly to have dual methods.

[lnicola]

Yes, I think we should use Buffer with easy conversions to ndarray. It's better than Vec because it has the shape information.

[metasim]

I'll prototype something.