API conventions cleanup

This PR performs several widespread renaming to bring a number of APIs into line with new conventions.

The details of the conventions can be found [here](http://discuss.rust-lang.org/t/settling-some-key-naming-conventions/269/35) and [in the guidelines](http://aturon.github.io/style/naming/README.html).

The most important changes are:

* The `unwrap` operation on `Option` and `Result` is now called `assert`.
* The `get_ref`, `get_mut_ref`, and `take_unwrap` methods on `Option` are deprecated.
* The standard trio of iterator functions are now `iter`, `iter_mut`, and `iter_owned`.
* More generally, `mut` variants universally use a suffix `_mut` rather than a prefix.

There were also a few more minor renamings.

Closes #13660 
Closes #13159
Closes #9784

All the original functions remain available in deprecated form. Nevertheless, this is considered a:

[breaking-change]