librustc: Implement the proc type as sugar for ~once fn and proc

notation for closures, and disable the feature gate for `once fn` if
used with the `~` sigil.

src/libsyntax/ast.rs

@@ -537,6 +537,7 @@ pub enum Expr_ {
     ExprLoop(Block, Option<Ident>),
     ExprMatch(@Expr, ~[Arm]),
     ExprFnBlock(fn_decl, Block),
+    ExprProc(fn_decl, Block),
     ExprDoBody(@Expr),
     ExprBlock(Block),