Guide Redux: Pointers
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.
auto merge of #15789 : steveklabnik/rust/guide_pointers, r=cmr

This is super, super WIP, but I'm going to go get lunch for a while, and figured I'd toss my work up here in case anyone wants to see my work as I do it.

This contains a new introductory section explaining the basics of pointers, and some pitfalls that Rust attempts to solve. I'd be interested in hearing how my explanation is, as well as if this belongs here. Pointers are such a crucial concept, I don't mind having a beginners' section on them in the main docs, even though our main audience is supposed to understand them already. Reasonable people may disagree, however.
Add support for labeled while loops.