Make set_indices a template

[romainbrenguier]

This makes the interface of goto_symex_statet simpler and can simplify
code of function templates using set_indices

This is based on #3986 (merged)

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[smowton]

Perhaps TODO isn't the right formulation, but the idea that we're supposed to be using the type system to track which expressions are renamed, and that reaching those TODOs means that doesn't fully happen yet (but we don't want to abruptly make that fatal) seems good to me.

[romainbrenguier]

@smowton

Perhaps TODO isn't the right formulation, but the idea that we're supposed to be using the type system to track which expressions are renamed, and that reaching those TODOs means that doesn't fully happen yet (but we don't want to abruptly make that fatal) seems good to me.

This comment should have been on #3986 (and @tautschnig already commented on that point)

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: f2e7514).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/103336929
Status will be re-evaluated on next push.
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 compatibility was already broken by an earlier merge.

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: eade946).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/103336871
Status will be re-evaluated on next push.
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 compatibility was already broken by an earlier merge.

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 78cf428).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/103368256
Status will be re-evaluated on next push.
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 compatibility was already broken by an earlier merge.