Further improve slice typecheck

[CAD97]

Follow-up to #83. There's two minor motivations to the change here:

• Protect against future changes to coercion/inference. By fully specifying the function pointer type, we can ensure that no coercion happens before the closure captures &#ty.
  • e.g. inserting an explicit coercion point with (|| &#ident as _) causes the current tyck to fail.
• Improve the error message by populating 'static lifetimes, making the type mismatch more directly comparable.

Also removes linkme::__private::value, as that helper function is no longer used.

The chance that a coercion could break the check done by #83 is reasonably low and would be caught on CI, and the error improvement is debatable at best, so it's a judgement call on whether these changes should be merged.

[dtolnay]

Thank you. I think the error is better like this.