Allow crate link attributes to contain nested meta items

[brson]

It's currently impossible to define a link attribute that has nested meta items, #[link(foo(bar))];, even though this is a valid way to write an attribute. There are unfinished branches in two methods, link::build_link_meta (that prevents translation), and attr::eq (that prevents the attribute from being matched in a use statement), that need to be filled in.

Unfortunately it's not possible to formulate a test case for this within the current test framework, so instead of writing potentially broken code, I've just made these scenarios fail.