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)
Would this remove all the ambiguity issues? If so I much prefer this. It unifies the two with a common sigil (and one less noisey than `#`), indicating that they do code generation, but they still show that they still have slightly different semantics.