auto merge of #20901 : dgrunwald/rust/update-token-can-begin-expr, r=sanxiyn

* add `Token::AndAnd` (double borrow)
 * add `Token::DotDot` (range notation)
 * remove `Token::Pound` and `Token::At`

This fixes a syntax error when parsing `fn f() -> RangeTo<i32> { return ..1; }`.

Also, remove `fn_expr_lookahead`.
It's from the `fn~` days and seems to no longer be necessary.

src/test/run-pass/range.rs

@@ -12,6 +12,9 @@
 
 fn foo() -> int { 42 }
 
+// Test that range syntax works in return statements
+fn return_range_to() -> ::std::ops::RangeTo<i32> { return ..1; }
+
 pub fn main() {
     let mut count = 0;
     for i in 0u..10 {