Add to_forall_expr [blocks: #3923]

[tautschnig]

There was a cast operation for exists, but none for forall.

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[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: 53819ac).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98541553
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

The incompatibility may have been introduced by an earlier PR. In that case merging this
PR should be avoided unless it fixes the current incompatibility.