Rework state wf: Add tag-unique-per-block, separate tree uniqueness and item wf

Sorry @neven I didn't know you were working on the same thing.

assigned to @neven

added 2 commits

• 778bce9e - 1 commit from branch iris:ci/tree-borrows
• 14c0856b - Rework state wf: Add tag-unique-per-block, separate tree uniqueness and item wf; overwrite Neven

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added 1 commit

• c478d9a4 - Further simplify state_wf, finish steps_op as far as feasible

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added 1 commit

• bbe189a4 - Refactor the tag map as discussed in mattermost

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added 1 commit

• bbacd47a - More progress in steps_opt

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added 2 commits

• 003f7771 - Strengthen write_becomes_active, make many deciders `Defined` to make testing possible
• 60fe6e76 - Add unit test to bor_semantics

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approved this merge request

I like the new definitions. Sorry that we had a bit of a conflict of working on the same thing.

mentioned in commit 7975ceb5

merged