String mutation (feature: unicode) is very slow

[s1341]

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