let_unit_value disallows `let () = ...;` when it is useful to ensure the type does not change

### Summary

An explicit `let () =` (or `let _: () =`) binding is used to guarantee that the return type of an API hasn't changed. This is useful to ensure that the API of an auto-generated C API doesn't change its return type to something that must be freed for example.

Basically if the unit type is explicitly specified, this lint shouldn't fire.

### Lint Name

let_unit_value