Remove basic updates from the Iris model, and define them using plainly.

We define basic updates as:

    |==> P  :=  (∀ Q, (P -∗ ■ Q) -∗ ■ Q)

From this definitions, we can prove all laws of basic updates, apart from
those related to frame preserving updates. For that, we need the following
primitive rule:

    x ~~>: Φ →
    uPred_ownM x ∗ (∀ y, ⌜Φ y⌝ -∗ uPred_ownM y -∗ ■ R) ⊢ ■ R.

So, in total, this gets rid of 1 primitive connective (|==>) and 5 primitive
rules (those of `|==>`), which is replaced by one new primitive rule.