Bundle type class for embedding.

This change is slightly more invasive than expected: in monPred we were
using the embedding before the BI was defined. With the new setup, this
is no longer possible, because in order to make an instance of the
embedding, we need to know that `monPred` is a BI. As such, we define
`emp`, `⌜ _ ⌝` and friends directly in the model of `monPred` and later
prove that they are equal to a version in terms of the embedding.