const param support

[steffahn]

currently, something like

#[self_referencing]
struct Foo<const C: usize> {
    x: (),
    #[borrows(x)]
    y: &'this (),
}

results in an unhelpful error message

error: custom attribute panicked
   --> src/main.rs:11:1
    |
 11 | #[self_referencing]
    | ^^^^^^^^^^^^^^^^^^^
    |
    = help: message: not implemented

[Y-jiji]

A quick patch. Please give me some unfriendly comments about soundness.