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/libcore/stackwalk.rs

@@ -94,7 +94,7 @@ pub mod rustrt {
 
 pub mod rusti {
     #[abi = "rust-intrinsic"]
-    pub extern {
+    pub extern "rust-intrinsic" {
         pub fn frame_address(f: &once fn(x: *u8));
     }
 }