Reduce the usage of `Felt::new_unchecked()`

After we've migrated to v0.23 VM (in https://github.com/0xMiden/protocol/pull/2931), it became clear that we are quite generous with our usage of `Felt::new_unchecked()` constructor. It would be good to go through the places where it is used, and check if we can replace this constructor with safer alternatives.