Explicitly use core hint database

Adding a hint without a database now triggers a deprecation warning in Coq master (https://github.com/coq/coq/pull/8987).

Thanks for the MR.

Could you let us know if this is backwards compatible with at least Coq 8.7?

Yup, as far as I know the behavior of a hint has always been to add it to core. (I also tested with 8.7 just to be sure.)

mentioned in commit 9fa4db07

merged

Merged, and thank you for the MR!