Add overload of join_string() that takes a function argument

[NlightNFotis]

Add overload of join_string() that takes a function argument that it applies to the elements of the container it flattens and add tests for it.

This is work that we did jointly @hannes-steffenhagen-diffblue as part of the work on the goto-harness feature.

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[tautschnig]

I believe the same can be achieved by doing auto range = make_range(container).map(transformer); join_strings(stream, range.begin(), range.end(), delim); Is that good or bad?

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: f98cbed).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99416416
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.