Add IntoPure instances for ∗, ∧, and ∨.

See merge request !49

Add a contributor's guide

See merge request !47

When using [iAssert ... with ">[]"], we should not use [tac_assert_persistent], and eliminate the modality instead.

This patch is still not ideal, because some modalities (e.g., later) preserve persistence.

For example, when having `"H" : ∀ x : Z, P x`, using
`iSpecialize ("H" $! (0:nat))` now works. We do this by first
resolving the `IntoForall` type class, and then instantiating
the quantifier.

Instead of doing all the instantiations by invoking a single type
class search, it now performs the instantiations by invoking
individual type class searches. This a.) gives better error messages
and b.) works when `xj` depends on `xi`.

In the following ways:
- When having `P → Q` it will now also work when the spatial context
  is non-empty.
- When having `∀ x : A, Q` it will now do an `iIntros (_)`.

Make it possible to apply the observational view shift lemmas.

See merge request !40

In this case, we cannot use all the hypotheses for proving the premises as well
as for the remaining goal.

use coq-stdpp

See merge request !46

It no longer requires the functions on both sides of the relation
to be syntactically the same.