OnDiskCorpus files be configurable to contain a human readable representation of the input

Most fuzzers will likely use some form of `OnDiskCorpus` (incl. `InMemoryOnDiskCorpus`, `CachedOnDiskCorpus`, etc.) for their solutions. To then figure out, what the problem actually was, one would need to know the content of the testcase/input that triggered the feedbacks. Currently, corpora storing them on disk store a bunch of generic information in the file associated with the testcase/input (such as runtime), but no representation of the input.

The only way to do add this without resorting to writing dummy-feedbacks that do nothing but add a new metadata with the input content, is by implementing the filename generating function on the input to extract the testcase from the corpus, and somehow stringify it:

```rust
fn generate_name(&self, id: Option) -> String;
```

However, file names have a length restriction, so this isn't usable for inputs that can get somewhat long. Plus, for structured inputs, it would be much easier to have the entire structure nicely formatted in the file.