syntax: Rewrite parsing of impls

Properly parse impls for the never type `!`
Recover from missing `for` in `impl Trait for Type`
Prohibit inherent default impls and default impls of auto traits
Change wording in more diagnostics to use "auto traits"
Some minor code cleanups in the parser

src/test/ui/span/impl-parsing.rs

@@ -0,0 +1,21 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -Z parse-only -Z continue-parse-after-error
+
+impl ! {} // OK
+impl ! where u8: Copy {} // OK
+
+impl Trait Type {} //~ ERROR missing `for` in a trait impl
+impl Trait .. {} //~ ERROR missing `for` in a trait impl
+impl ?Sized for Type {} //~ ERROR expected a trait, found type
+impl ?Sized for .. {} //~ ERROR expected a trait, found type
+
+default unsafe FAIL //~ ERROR expected `impl`, found `FAIL`