Fill in example code for HashMap.

Add an example showing how to use the map with a custom type. Fill in
examples for methods in the hashmap file without ones.

Also move pop_equiv next to related public methods, to not create a
duplicate trait implementation in the docs.
Remove explicit rust code specifier. Unhide use HashMap.
Cleanup HashMap documentation.

Link to mentioned methods. Use `# Failure` tags to describe failure.
Make `pop_equiv`, `find_equiv` and `get_copy` standalone.
auto merge of #15787 : treeman/rust/hashmap-doc, r=alexcrichton

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.