RFC: Resolve numerous naming conventions

This is a conventions RFC for settling a number of remaining naming conventions:

* Referring to types in method names
* Iterator type names
* Getter/setter APIs
* Associated types
* Trait naming
* Lint naming
* Suffix ordering

It also proposes to standardize on lower case error messages within the compiler
and standard library.

[Rendered](https://github.com/aturon/rfcs/blob/conventions-galore/active/0000-conventions-galore.md)


`Show` doesn't quite fit into this category because its method is called `fmt` due to sticking with the other formatting traits. (not sure if that affects this convention)