A set of assertion-related functions has been introduced:

* `revng_abort(message)`: aborts, in release builds too.
* `revng_check(what, message)`: asserts `what`, in release builds
  too. Also emits a `__builtin_assume`, that can lead to additional
  optimizations in clang.
* `revng_unreahcable(message)`: identical to `revng_abort`, but in
  release builds emits `__built_unreachable`.
* `revng_assert(what, message)`: asserts in debug builds, otherwise
  emits `sizeof(what)` (to suppress unused variable warnings) and
  `__builtin_assume`.

The adoption of these function has the following benefits:

* Nice stack traces.
* The developer can choose to enforce an `assert` (or an `unreachable`)
  at release-time too by using `check`/`abort`.
* Most warnings about unused variables in release mode should be gone.
* When using clang, the `assert`s become `assume`s, which might enable
  additional optimizations (with no run-time costs).
* The `assert(Condition && "Reason")` trick is no longer needed, we now
  have a proper argument.