`CopySlice` simplification, more generic Array/Map slot simplification, fixing `decompose`, `--nodecompose` flag

[zoep]

Description

Changes:

• add simplification of CopySlice so that the output of hevm is closer to the Expr output of Act. Expressions of the form CopySlice 0 0 size src (ConcreteBuf "") are rewritten to src when size is equal to the size of the src buffer`.
• CopySlice simplification necessitated improvement of both decomposition and readStorage improvements
• Decomposition of storage can now be disabled on the command line with --nodecompose
• decompose can safely skip writes to an idx it's not currently reading. This is because in decompose we have a very good idea of e.g. no mixing of map/array/other and we know exactly what idx is being read. So we have the safeguards necessary to do this.
• decompose is now only safe to execute in case everything safely decomposes. This is actually a requirement for AbstractStore k, as separate queries will deal with the same AbstractStore k. Aalthough we could be more lenient and have it decompose for every AbstractStore k1 and not for AbstractStore k2 and still be OK to decompose AbstractStore k1 and not AbstractStore k2. However, the strict one works for now.
• We cannot decompose when it's PEq of the whole storage -- not with the current setup. We'd need a more sophisticated decompose for that. That kind of check only occurs during equivalence checking. We now check for this and don't perform decomposition on such queries (rare).

Checklist

• [X] tested locally
• [x] added automated tests
• [ ] updated the docs
• [x] updated the changelog

[msooseth]

I have now fixed this PR I think, but the more general system that I now created needs a review. Furthermore, I am not sure which order should the dop/take be, that needs to be checked.

[msooseth]

Thanks for the review @arcz ! Let me know if you think it's OK now :)