Handle IDENT restrictions

[ehuss]

Currently the grammar allows restricted identifiers in IDENT. However, libsyntax does not. parse_ident_common

The following are examples rejected by libsyntax:

mod _ {}
mod as {}
mod abstract {}
mod self {}

The restriction should include all strong and reserved keywords, and _.

_ is special, it is allowed in a few places:

Where	Example
const item	const _: i32 = 0;
extern crate as alias	extern crate c as _;
use aliases	use x as _;
extern block function parameter names	extern { fn f(_: i32); }
function pointer type	fn(_: i32)

self is allowed for extern crate self as foo;

Paths allow several keywords: super, self, Self, and crate.