Asynchronous executors

[addisoncrump]

This PR introduces asynchronous executors will task batching for use with asynchronous execution environments. This enables many new use cases, including (but not limited to):

• distributed executors (fuzzer pipeline hosted on a single machine, executors distributed over multiple)
• differential execution over diverse platforms (e.g., diff fuzzing a macOS target and a Windows target)
• use of std fuzzer pipelines with no-std environments (e.g., low-level fuzzing with testcases submitted over a channel)

The process for performing asynchronous fuzzing prefers task batching. This is done using DeferredExecutionResult and expects each stage to offer a batching execution where possible, e.g. for a mutational stage:

1. Determine how many mutations will be created (n)
2. Allocate space for n DeferredExecutionResults
3. For each mutation:
   1. Mutate the input
   2. Start the execution on the executor and stash the DeferredExecutionResult
4. For each DeferredExecutionResult:
   1. Get the result
   2. Check the interestingness with the associated feedback

Because a blanket impl of AsyncExecutor exists for all Executors with observers, synchronous executors may be used in an asynchronous pipeline at the cost of an extra clone of the observers. Additionally, AsyncBridge may be used to use asynchronous executors in a synchronous pipeline, but may suffer execution speed due to being used without batching.

[domenukk]

Cc @epi052 you may like this

[addisoncrump]

Well, the CI is broken, but I've tested the fuzzers locally and they do be worky 👍

[addisoncrump]

This is still missing a lot of perf items and such which are likely desirable for introspection. Working on that still, but it's in a semi-stable state.

[addisoncrump]

Stages that need async support (check is complete, cross-out is deemed impossible):

• CalibrationStage
• ClosureStage (by definition compatible)
• ConcolicTracingStage
• DumpToDiskStage (by definition compatible)
• GeneralizationStage
• PowerMutationalStage (batched executions, collect results later)
• PushStageAdapter
• SimpleConcolicMutationalStage
• SkippableStage (by definition compatible)
• StdMutationalStage (batched executions, collect results later)
• StdTMinMutationalStage
• SyncFromDiskStage
• TracingStage
• TuneableMutationalStage
• ShadowTracingStage

Other things that need doing:

• DifferentialExecutor
• DifferentialObserver

[domenukk]

Is this ready to merge?

[addisoncrump]

No, I need to complete the checklist above.

[domenukk]

Closing this, a lot has happened since. Feel free to reopen if it's still relevant.