Add support for prefix unary operators.

E.g. if you add `@` to `parse.nolhs`,

    a b @c d

will be equivalent to

    a b (@c) d
Multiply all link precedences by 10.

Now I have some space to add new ones.
Slightly lower precedence of `->`.

That way `a.b -> c` parses as `(a.b) -> c`. `a.(b -> c)` doesn't make sense anyway.