librustc: Implement the syntax in the RFC for unboxed closure sugar.

Part of issue #16640. I am leaving this issue open to handle parsing of
higher-rank lifetimes in traits.

This change breaks code that used unboxed closures:

* Instead of `F:|&: int| -> int`, write `F:Fn(int) -> int`.

* Instead of `F:|&mut: int| -> int`, write `F:FnMut(int) -> int`.

* Instead of `F:|: int| -> int`, write `F:FnOnce(int) -> int`.

[breaking-change]

src/libsyntax/ast.rs

@@ -213,12 +213,19 @@ pub static DUMMY_NODE_ID: NodeId = -1;
 #[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash, Show)]
 pub enum TyParamBound {
     TraitTyParamBound(TraitRef),
-    UnboxedFnTyParamBound(UnboxedFnTy),
+    UnboxedFnTyParamBound(P<UnboxedFnBound>),
     RegionTyParamBound(Lifetime)
 }
 
 pub type TyParamBounds = OwnedSlice<TyParamBound>;
 
+#[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash, Show)]
+pub struct UnboxedFnBound {
+    pub path: Path,
+    pub decl: P<FnDecl>,
+    pub ref_id: NodeId,
+}
+
 #[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash, Show)]
 pub struct TyParam {
     pub ident: Ident,