Improve array len fuzzing

[juntyr]

And yet another attempt at fixing array len fuzzing so we won't overload the fuzzer with OOM or timeouts. Now, the array length is produced like a Vec len but only once for the first instantiation.

To ensure that [[(); N]; M] costs the fuzzer NxM units of input instead of just N+M, the arbitrary value production now imposes a minimal penalty on unit/empty values. While this wastes some input, it (hopefully) ensures that the fuzzed values do not explode, since they are bounded by the input.

* [ ] I've included my change in CHANGELOG.md

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (e3da023) 98.14% compared to head (c9670ca) 98.14%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #495 +/- ## ======================================= Coverage 98.14% 98.14% ======================================= Files 78 78 Lines 10308 10308 ======================================= Hits 10117 10117 Misses 191 191 ```

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