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also add `wand_entails'`, which was used in some earlier version of these proofs

Thank you, Robbert.

Add lemmas `big_sepM2_delete_{l,r}` and rename `big_sepM2_lookup_{1,2}` into `big_sepM2_lookup_{l,r}`.

Hence, also rename (the old) `equiv_entails` → `equiv_entails_1_1` and
`equiv_entails_sym` → `equiv_entails_1_2`, and add `equiv_entails_2` for completeness.

Generalize `big_opM_filter'` and `big_opS_filter'` to arbitrary monoids, and make arguments consistent.