Revert "Use std::forward_list instead of std::map in irept by default"

[tautschnig]

This is an experiment to see whether NAMED_SUB_IS_FORWARD_LIST=1 is the
root cause of builds and tests taking >70 minutes on Travis when
previously they would only take 26 minutes. (confirmed)

This is now a clean revert of the second commit in #4458.

• 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).
• 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]

It seems that running the regression tests on Travis with NAMED_SUB_IS_FORWARD_LIST set takes a lot longer. This is not the expected behaviour, but while I investigate this we should just go back to the known-good state.

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: be29a84).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/113681045
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: 4d020ce).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/113682654
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.