move string solver into separate directory

[kroening]

• Each commit message has a non-empty body, explaining why the change was made.
• n/a 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
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[tautschnig]

This seems to be missing a git add, the files have just been removed, not re-added. And likely it will need a clang-format with results to go in a separate commit to make CI happy.

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: 46e2151).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/97658054
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.

[majakusber]

Rebased and updated the test-gen pointer bump. Once both pass CI, I'll merge this.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 3762599).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98022743

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 93a0994).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98064757