Store identifiers of parameters in goto_functiont::parameter_identifiers [blocks: #4167]

[kroening]

This is to enable a transition towards #4167.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 548f19a).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/101191642

[tautschnig]

Just throwing a blocking review in there to buy myself some time to add a few tweaks from #4167. I'll take care of this, no action on anyone else's part required.

[tautschnig]

I have amended the commit to include all changes necessary to make goto_functiont::parameter_identifiers fully usable.

[hannes-steffenhagen-diffblue]

Mostly lgtm

[hannes-steffenhagen-diffblue]

⛏️ ⛏️ I have a small preference for using <algorithm> (std::transform in this case) where applicable

[tautschnig]

What would a concise piece of code achieving this look like? I'd like to have a list comprehension like parameter_identifiers = [p.get_identifier() for p in code_type.parameters()]; - but that's Python, not C++...