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

[dgrunwald]

• 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.

[rust-highfive]

r? @sfackler

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[alexcrichton]

Would it be possible to add regression tests for the added cases as well?

[dgrunwald]

I added a test case for range notation.
The test case for double-borrow would be

fn f() -> &'static &'static i32 {
   return &&1;
}

but that's currently a lifetime error (but I think @eddyb wants to change that).