Update Token::can_begin_expr() to make it consistent with the grammar:

Update Token::can_begin_expr() to make it consistent with the grammar:
 * add Token::AndAnd (double borrow)
 * add Token::DotDot (range notation)
 * remove Token::Pound and Token::At

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.