Fill in example code for HashMap.

Add an example showing how to use the map with a custom type. Fill in
examples for methods  without ones.

Also move `pop_equiv` next to related public methods, to not create a
duplicate trait in the docs.
The docs should stand somewhat on their its own (at least, there should be a relatively specific summary), could this be adjusted to include something like [the description of `pop`](http://doc.rust-lang.org/master/std/collections/trait.MutableMap.html#tymethod.pop)?