Fix some grammar inconsistencies for the '..' range notation.

Grammar changes:
* allow 'for _ in 1..i {}' (fixes #20241)
* allow 'for _ in 1.. {}' as infinite loop
* prevent use of range notation in contexts where only operators of high
  precedence are expected (fixes #20811)

Parser code cleanup:
* remove RESTRICTION_NO_DOTS
* make AS_PREC const and follow naming convention
* make min_prec inclusive

src/test/compile-fail/range-3.rs

@@ -0,0 +1,16 @@
+// Copyright 2014 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.
+
+// Test range syntax - syntax errors.
+
+pub fn main() {
+    let r = 1..2..3;
+    //~^ ERROR expected one of `.`, `;`, or an operator, found `..`
+}

src/test/compile-fail/range-4.rs

@@ -0,0 +1,16 @@
+// Copyright 2014 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.
+
+// Test range syntax - syntax errors.
+
+pub fn main() {
+    let r = ..1..2;
+    //~^ ERROR expected one of `.`, `;`, or an operator, found `..`
+}

src/test/run-pass/ranges-precedence.rs

@@ -48,5 +48,12 @@ fn main() {
     assert!(x == &a[3..]);
 
     for _i in 2+4..10-3 {}
+
+    let i = 42;
+    for _ in 1..i {}
+    for _ in 1.. { break; }
+
+    let x = [1]..[2];
+    assert!(x == (([1])..([2])));
 }