rename assert! and assert_eq! to reflect that they uphold an invariant

These are not traditional assertions used for sanity checking that are optimized out in release builds. There are cases such as the asserts in `std::cell::RefCell` that are performing a useful sanity check, but should not be in a release build. By renaming these, we can make room for more traditional assertions.

I would suggest `enforce!(condition)` and `enforce_eq!(a, b)`.
