auto merge of #16689 : wickerwaka/rust/crate-as, r=pcwalton

For review. Not sure about the link_attrs stuff. Will work on converting all the tests.

extern crate "foobar" as foo;
extern crate foobar as foo;

Implements remaining part of RFC #47.
Addresses issue #16461.

Removed link_attrs from rust.md, they don't appear to be supported by
the parser.

diff --cc src/libsyntax/parse/parser.rs
Simple merge

diff --cc src/libsyntax/print/pprust.rs
Simple merge

diff --cc src/test/pretty/issue-4264.pp
Simple merge