Fully rewrite nested forall during symbolic execution

[tautschnig]

This permits (correctly) solving another simple regression test.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 9351d80).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98787444

[martin-cs]

Given the improving quality of quantification over bit-vectors, we might want to keep half a mind towards preserving quantifier structure where we can.