Bundle classes for updates, plainly and embeddings

See merge request FP/iris-coq!125

Based on an earlier MR by @jung.

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.

Get rid of `FromLaterN` class.

See merge request FP/iris-coq!123

Make iFrame able to accumulate assertions in an evar.

See merge request FP/iris-coq!124

This requires changing the Hint Mode of the [Frame] type class because it should not fail if its parameter is an evar, but instantiate it instead. In order to prevent all the other instances of [Frame] to intantiate this evar themselves, we create a new type class [KnwonFrame], which corresponds to the old behavior.

See the discussion in #163.

Super `iModIntro` tactic that generalizes `iAlways`, `iNext`, `iModIntro`, and more

See merge request FP/iris-coq!121