Unify attributes and macros to use `@` sigil, redux

A revised version of @pcwalton's RFC, incorporating a more precise description of conflicts and a description of feedback.

[Rendered view.](https://github.com/nikomatsakis/rfcs/blob/macros-and-attrs/active/0000-macro-attribute-syntax.md)
I think `@foo:` rather than `@[foo]` would also resolve the ambiguity