Add AbiSet and integrate it into the AST.

I believe this patch incorporates all expected syntax changes from extern
function reform (#3678). You can now write things like:

    extern "<abi>" fn foo(s: S) -> T { ... }
    extern "<abi>" mod { ... }
    extern "<abi>" fn(S) -> T

The ABI for foreign functions is taken from this syntax (rather than from an
annotation).  We support the full ABI specification I described on the mailing
list.  The correct ABI is chosen based on the target architecture.

Calls by pointer to C functions are not yet supported, and the Rust type of
crust fns is still *u8.

src/libsyntax/ast_util.rs

@@ -465,7 +465,7 @@ pub fn id_visitor(vfn: @fn(node_id)) -> visit::vt<()> {
                     vfn(self_id);
                     vfn(parent_id.node);
                 }
-                visit::fk_item_fn(_, generics, _) => {
+                visit::fk_item_fn(_, generics, _, _) => {
                     visit_generics(generics);
                 }
                 visit::fk_method(_, generics, m) => {