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/librustpkg/rustpkg.rc

@@ -185,7 +185,7 @@ impl PackageScript {
         // the build API.
         for crate.node.module.items.each |i| {
             match i.node {
-                ast::item_fn(_, _, _, _) => {
+                ast::item_fn(*) => {
                     custom = true;
 
                     break;