Use model for String.hashCode

[romainbrenguier]

The version in the solver is incomplete (under-constrained) and it is not possible to have a complete version which would be really language independent, so it is better to implement these in models, like we do in diffblue/java-models-library#10 for Java

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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).
• [na] My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• [na] White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 56e097e).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/108816446
Status will be re-evaluated on next push.
Common spurious failures include: 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); compatibility was already broken by an earlier merge.

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: b6c30ba).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/108857635
Status will be re-evaluated on next push.
Common spurious failures include: 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); compatibility was already broken by an earlier merge.

[tautschnig]

@romainbrenguier What's the state of this?

[romainbrenguier]

@tautschnig

@romainbrenguier What's the state of this?

I'm waiting on a test-gen pull request to confirm this is safe to merge, but it doesn't compile with the current version of cbmc at the moment.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 7353640).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/111548598

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 18d1a00).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/111562482
Status will be re-evaluated on next push.
Common spurious failures include: 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); compatibility was already broken by an earlier merge.