RFC: Macro reform

[Rendered view](https://github.com/kmcallister/rfcs/blob/macro-reform/text/0000-macro-reform.md)
Yeah, I don't expect it to be a common situation.  One example would be quasiquoting as a library.  `quote_foo!()` expands to calls into `syntax::ext::quote::rt::...`.  And there's no need for this to live in a separate crate from the quote macro, because the user of quasiquoting is also going to link librustc, or libsyntax anyway.

Basically it's a capability we have today with `#[phase(plugin, link)]` and I didn't want to throw it away.  Though, the `Registry` API is unstable and we'd only be adding capabilities to it, so maybe we *should* throw this out for the time being and see if anyone complains.  It might be good to nudge people towards doing thing the two-crates way.