proposal: two-dimensional array slices

[hades]

Proposal: support for Array2d slices.

Array2d is a very useful type for two-dimensional data storage. I've used
it extensively while solving Advent of Code and Everybody Codes puzzles[1].

However, one feature that I wish the crate provided was array "views", i.e.
smaller rectangular subarrays that I could borrow into a function without
copying an entire (large) array.

This would function similarly to a (one-dimensional) Rust slice &[T].

I propose an API similar to Array2D itself, so that in many cases this new
type can be used as a drop-in replacement. In particular, the following
functions will be supported:

• num_rows, num_columns, num_elements, row_len, column_len
• get
• row_iter, column_iter
• rows_iter, columns_iter
• index
• eq, ne

For mutable slices also:

• get_mut
• set
• index_mut

Additionally, a copy of an array can be constructed with an as_array method.

In [2] I've cobbled together a very simple implementation of an ArrayView for
reference on how such a type could be useful.

[1] https://github.com/hades/everybodycodes
[2] https://github.com/hades/everybodycodes/blob/master/src/quest10.rs#L35