Parse 2.0 macro syntax

[dtolnay]

The current syntax looks like this.

pub macro run_passes($tcx:ident, $mir:ident, $def_id:ident, $suite_index:expr; $($pass:expr,)*) {{
    /* same as usual */
}}

[dtolnay]

I implemented the bare minimum to pass tests in 500d832, will need to revisit what these look like in the AST.

[mystor]

I think that your implementation is fine for now. We can come back to it & clean up the AST once the AST used by rustc stabilizes some more.

[dtolnay]

I kept the AST but restricted the grammar in e082403 -- the arguments must be parenthesized and the body must be braced.