Add helper types for #1350

[RobinTF]

Breakout part of #1350

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.39456% with 20 lines in your changes missing coverage. Please review.

Project coverage is 89.61%. Comparing base (3fad814) to head (bb3f0c4).

Files	Patch %	Lines
src/util/CacheableGenerator.h	86.23%	1 Missing and 18 partials :warning:
src/util/IteratorWrapper.h	88.88%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1435 +/- ## ========================================== - Coverage 89.63% 89.61% -0.03% ========================================== Files 343 345 +2 Lines 29951 30098 +147 Branches 3315 3341 +26 ========================================== + Hits 26847 26972 +125 - Misses 1957 1958 +1 - Partials 1147 1168 +21 ```

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[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud