RFC: Convention: do no prefix exports with the module's name

This is a conventions RFC that proposes that the items exported from a module
should *never* be prefixed with that module name. For example, we should have
`io::Error`, not `io::IoError`.

(An alternative design is included that special-cases overlap with the
`prelude`.)

[Rendered](https://github.com/aturon/rfcs/blob/no-module-prefixes/active/0000-no-module-prefixes.md)