[WIP] Make cache mutable struct

[Vaibhavdixit02]

Checklist

• [ ] Appropriate tests were added
• [ ] Any code changes were done in a way that does not break public API
• [ ] All documentation related to code changes were updated
• [ ] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• [ ] Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (35b97e5) 8.93% compared to head (2a893c3) 1.95%. Report is 36 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #674 +/- ## ========================================= - Coverage 8.93% 1.95% -6.98% ========================================= Files 31 31 Lines 2506 2501 -5 ========================================= - Hits 224 49 -175 - Misses 2282 2452 +170 ```

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