Add lemmas for creating later credits for non-pure steps (alternative)

See merge request iris/iris!1036

Fix #566 by introducing IsDisjUnion class

Closes #566

See merge request iris/iris!1039

Prevent iFrame from instantiating existentials under ∀, -∗, →, WP

Closes #565

See merge request iris/iris!1035

Add `twp_wp_step_lc` and use that to prove `steps_lb` later credit lemmas for HeapLang atomic instructions.

always do Proof. after Restart.

See merge request !1040

frame_impl: explain why we do not use QuickPersistent

Closes #563

See merge request !1038

Add lemma `lc_fupd_add_laterN`.

See merge request iris/iris!1037

Proof by @tlsomers

update test-normalizer: remove old cruft, add Ltac2 printing change

See merge request iris/iris!1033

Improve `iFrame` to instantiate existential quantifiers

See merge request !1017