syntax: Implement #![no_core]

This commit is an implementation of [RFC 1184][rfc] which tweaks the behavior of
the `#![no_std]` attribute and adds a new `#![no_core]` attribute. The
`#![no_std]` attribute now injects `extern crate core` at the top of the crate
as well as the libcore prelude into all modules (in the same manner as the
standard library's prelude). The `#![no_core]` attribute disables both std and
core injection.

[rfc]: https://github.com/rust-lang/rfcs/pull/1184

src/libsyntax/test.rs

@@ -258,6 +258,7 @@ fn generate_test_harness(sess: &ParseSess,
         config: krate.config.clone(),
         toplevel_reexport: None,
     };
+    cx.ext_cx.crate_root = Some("std");
 
     cx.ext_cx.bt_push(ExpnInfo {
         call_site: DUMMY_SP,