Reversed arbitrary

Hello,

I've been toying with custom mutators in cargo-fuzz with libfuzzer and it seems to me to support changes to typed data instead of bytes, the reversed arbrirtary operation would be really helpful. Here's the interface I have implemented so far:

/// Define a custom fuzz mutator.
///
/// If `$bytes` exceeds `$max_size`, it will be silently truncated.
///
/// ## Example
/// ```no_run
/// #![no_main]
/// use libfuzzer_sys::{fuzz_target, fuzz_mutator, llvm_fuzzer_mutate};
///
/// fuzz_target!(|data: &[u8]| {
///     let _ = std::str::from_utf8(data);
/// });
///
/// fuzz_mutator!(|data: &mut [u8], max_size: usize| {
///     println!("custom mutator called with data len = {} and max_size = {}", data.len(), max_size);
///     /* call wrapper function of libfuzzer's default mutator */
///     llvm_fuzzer_mutate(data, max_size)
/// });
/// ``` 

The mutator with typed data would be the equivalent:

fuzz_mutator!(|data: &mut T, max_size: usize | {
   loop {
   /* perform changes on `data` */

   if data.arbitrary_size() <= max_size { /* where arbitrary_size is some method from the Trait to calculate size in bytes */
     break;
  }
}
 });

Does this sound like a reasonable approach to you?

No file or test is named. Start by reviewing the custom fuzz mutator interface and the proposed typed-data example, then determine whether a reversed arbitrary operation fits the existing API; done would require an agreed design and corresponding implementation and tests.