implement the ? operator

The `?` postfix operator is sugar equivalent to the try! macro, but is more amenable to chaining:
`File::open("foo")?.metadata()?.is_dir()`.

`?` is accepted on any *expression* that can return a `Result`, e.g. `x()?`, `y!()?`, `{z}?`,
`(w)?`, etc. And binds more tightly than unary operators, e.g. `!x?` is parsed as `!(x?)`.

cc #31436

src/libsyntax/print/pprust.rs

@@ -2277,6 +2277,10 @@ impl<'a> State<'a> {
                 try!(self.print_inner_attributes_inline(attrs));
                 try!(self.print_expr(&e));
                 try!(self.pclose());
+            },
+            ast::ExprKind::Try(ref e) => {
+                try!(self.print_expr(e));
+                try!(word(&mut self.s, "?"))
             }
         }
         try!(self.ann.post(self, NodeExpr(expr)));