String mutation (feature: unicode) is very slow

**IMPORTANT**
1. You have verified that the issue to be present in the current `main` branch

Yup

**Describe the bug**
Building a real fuzzer with the `String*` mutations results in a very slow fuzzer.

```
Client 001:
0.0022: Scheduler
0.0361: Manager
Stage 0:
0.0002: GetInputFromCorpus
0.8122: Mutate
0.0002: MutatePostExec
0.0713: TargetExecution
0.0012: PreExecObservers
0.0120: PostExecObservers
Feedbacks:
0.0000: CrashFeedback
0.0289: mapfeedback_metadata_edges
0.0000: time
0.0357: Not Measured
Client 002:
0.0020: Scheduler
0.0385: Manager
Stage 0:
0.0006: GetInputFromCorpus
0.8167: Mutate
0.0002: MutatePostExec
0.0654: TargetExecution
0.0011: PreExecObservers
0.0111: PostExecObservers
Feedbacks:
0.0000: CrashFeedback
0.0268: mapfeedback_metadata_edges
0.0000: time
0.0378: Not Measured
```