Seal the `IntoIdentifiedGitOid` trait.

[alilleybrinker]

IntoIdentifiedGitOid is the trait used in the gitoid crate to enumerate the types which can be used to construct a GitOid finder. This is a useful mechanism for providing a generic and ergonomic method for constructing the finder, but it should be sealed to prevent extension by third-parties, since incorrect extensions would result in invalid GitOids.

[alilleybrinker]

This will also have the benefit of making the FFI story for this API easier: just provide a unique function for each of the generic variants, since it's a finite set of types.

[alilleybrinker]

This trait has been removed!