Move {json,xml}_expr.{h,cpp} to goto-programs [blocks: #3734, #3736]

[tautschnig]

This prepares the move of {json,xml}_expr.{h,cpp} to goto-programs to remove the
dependency of util on langapi.

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• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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• My PR is restricted to a single feature or bugfix.
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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: e6d5e21).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/96535289

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: bf25f8d).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/96539054
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.

[tautschnig]

I suspect this requires a genuine update in TG as quite possibly it uses util/json_expr.h (which should now be goto-programs/json_expr.h. Could a bump please be created @allredj @peterschrammel @thk123?

[antlechner]

@tautschnig It does look like this needs a TG update, I'm working on it and I'll keep you updated.

[tautschnig]

Thank you @antlechner! I have rebased this PR to hopefully simplify the TG work.

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: 833721d).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/96755542
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.

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: d8b511f).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/96772500
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.

[antlechner]

@tautschnig We have a TG pointer bump that passes now, so feel free to merge this PR. Thanks for notifying us that an update was needed. 🙂

[tautschnig]

Thanks @antlechner! Re-assigning to @kroening and @peterschrammel for review.